If condition problem

Hi all, I am building a code that does a similar job like MATCH INDEX or Vlookup.
but these functions need their inputs to be in an certain order if i want a APPROX match.
so I built my own code.
I worked for a while and now it doesn't anymore...

here is my condition that gives me headache

Sub MPApproximation()
'On Error Resume Next
    'Application.ScreenUpdating = False
    'Application.DisplayAlerts = False
Dim Difference As Double
Dim DifferenceBefore As Double
Dim DifferenceAfter As Double


For i = 4 To 150
    Range("Y" & i).Value = RowBefore

RowBefore = -1
RowAfter = -1
DifferenceAfter = 1000
DifferenceBefore = 1000
'For l = 4 To 10000
'MPReference = Range("O" & l).Value
'If MPReference - LastMPReference > 5 Then
'Newsection = l
'End If
LastMPReference = MPReference

    For j = 4 To 2000
        Difference = (Range("Q" & i).Value - Range("O" & j).Value)
        If Diffenrence <= 0 And Abs(Difference) < Abs(DifferenceBefore) And j >= RowBefore And j < RowBefore + 100 Then
                DifferenceBefore = Difference
                RowBefore = j
                'MsgBox DifferenceBefore
            End If
            
'        If Difference >= 0 And Abs(Difference) < Abs(DifferenceAfter) And j >= 127 Then
'                DifferenceAfter = Difference
'                RowAfter = j
'                'MsgBox DifferenceAfter
'            End If
            
        'If Difference = 0 Then
       ' RowAfter = j
       ' RowBefore = j
       ' MsgBox Difference
       ' End If
    Next j
    
    'MsgBox RowBefore
    'MsgBox Rowafter
    
'    LatAfter = Range("G" & RowAfter).Value
'    LongAfter = Range("H" & RowAfter).Value
    LatBefore = Range("G" & RowBefore).Value
    LongBefore = Range("H" & RowBefore).Value
'    MPAfter = Range("O" & RowAfter).Value
    MPBefore = Range("O" & RowBefore).Value
    
'    Range("R" & i).Value = MPAfter
'    Range("S" & i).Value = LatAfter
'    Range("T" & i).Value = LongAfter
    Range("U" & i).Value = MPBefore
    Range("V" & i).Value = LatBefore
    Range("W" & i).Value = LongBefore
    Range("X" & i).Value = RowBefore

    'Range("Y" & i).Value = DifferenceAfter >= 0
    'Range("Z" & i).Value = DifferenceBefore <= 0

Next i

    For k = 4 To 150
    If Range("X" & k).Value = False Then
    Range("AA" & k).FormulaR1C1 = _
        "=((RC[-5]-RC[-8])/(RC[-6]-RC[-9]))*(RC[-10]-RC[-9])+RC[-8]"
    Range("AB" & k).FormulaR1C1 = _
        "=((RC[-5]-RC[-8])/(RC[-7]-RC[-10]))*(RC[-11]-RC[-10])+RC[-8]"
    Else
    Range("AA" & k).Value = Range("S" & k).Value
    Range("AB" & k).Value = Range("T" & k).Value
        End If
        
    Next k
    Application.ScreenUpdating = True
    Application.DisplayAlerts = True
    MsgBox "lol"
End Sub

I am debugging it, so there might be some wierd stuff and stuff that are commented. but the real headache part is here:

        If Diffenrence <= 0 And Abs(Difference) < Abs(DifferenceBefore) And j >= RowBefore And j < RowBefore + 100 Then

first and second statement works fine, at the third, it doesn't here is sample result:

622 621
622 622
165 622
165 165
165 165
168 165
170 168

(these are j value, for loop iteration, or row number)

Some BG story.
Its a code to approximate a Milepost depending on the GPS location. the mile post has been approximated depeding on the distance different points has. but the data acquisition is every 10 seconds. so there are holes and we want a more "accurate" approx. so I built something that gets the two closest points (over and under the searching value). after i simply extrapolate the value and search it via the wanted value. the prob is j is not comparing properly..

thank you all!

Well I think I sort of solved the problem by adding a new variable instead of rowafter and rowbefore.

But I still have a problem with the comparison with the doubles.
I tried to round them to 10 decimals, but it seems like it doesn't work.
any help?

here is the updated code :

Sub MPApproximation()
'On Error Resume Next
'    Application.ScreenUpdating = False
'    Application.DisplayAlerts = False
Dim Difference As Double
Dim DifferenceBefore As Double
Dim DifferenceAfter As Double

RowBefore = 0
RowAfter = 0

For i = 4 To 150
    RowBefore2 = RowBefore
    RowAfter2 = RowAfter
RowBefore = -1
RowAfter = -1
DifferenceAfter = 1000
DifferenceBefore = 1000
'For l = 4 To 10000
'MPReference = Range("O" & l).Value
'If MPReference - LastMPReference > 5 Then
'Newsection = l
'End If
LastMPReference = MPReference

    For j = 4 To 2000
        Difference = (Range("Q" & i).Value - Range("O" & j).Value)
        RoundedDifference = Round(Difference, 10)
        If Diffenrence <= 0 Then
        If j >= RowBefore2 And Abs(RoundedDifference) < Abs(RoundedDifferenceBefore) And j <= RowBefore2 + 150 Then
        Wait 0.001
                DifferenceBefore = RoundedDifference
                RoundedDifferenceBefore = Round(DifferenceBefore, 10)
                RowBefore = j
                'MsgBox DifferenceBefore
            End If
            End If
            
        If Difference >= 0 Then
        If j >= RowAfter2 And Abs(RoundedDifference) < Abs(RoundedDifferenceAfter) And j <= RowAfter2 + 150 Then
                DifferenceAfter = RoundedDifference
                RoundedDifferenceAfter = Round(DifferenceAfter, 10)
                RowAfter = j
                'MsgBox DifferenceAfter
            End If
            End If
            
        'If Difference = 0 Then
       ' RowAfter = j
       ' RowBefore = j
       ' MsgBox Difference
       ' End If
    Next j
    
    'MsgBox RowBefore
    'MsgBox Rowafter
    
    LatAfter = Range("G" & RowAfter).Value
    LongAfter = Range("H" & RowAfter).Value
    LatBefore = Range("G" & RowBefore).Value
    LongBefore = Range("H" & RowBefore).Value
    MPAfter = Range("O" & RowAfter).Value
    MPBefore = Range("O" & RowBefore).Value
    
    Range("R" & i).Value = MPAfter
    Range("S" & i).Value = LatAfter
    Range("T" & i).Value = LongAfter
    Range("U" & i).Value = MPBefore
    Range("V" & i).Value = LatBefore
    Range("W" & i).Value = LongBefore
    Range("X" & i).Value = RowBefore = RowAfter

    'Range("Y" & i).Value = DifferenceAfter >= 0
    'Range("Z" & i).Value = DifferenceBefore <= 0

Next i

    For k = 4 To 150
        If Range("X" & k).Value = False Then
            Range("AA" & k).FormulaR1C1 = _
                "=((RC[-5]-RC[-8])/(RC[-6]-RC[-9]))*(RC[-10]-RC[-9])+RC[-8]"
            Range("AB" & k).FormulaR1C1 = _
                "=((RC[-5]-RC[-8])/(RC[-7]-RC[-10]))*(RC[-11]-RC[-10])+RC[-8]"
        Else
            Range("AA" & k).Value = Range("S" & k).Value
            Range("AB" & k).Value = Range("T" & k).Value
        End If
    Next k
    
    
    Application.ScreenUpdating = True
    Application.DisplayAlerts = True
End Sub