Convert a worksheet formula to a function

In order to rationalise a weekly average to take account that one unit may not be considered a full unit for division purposes (ie if a units revenue is 50% of target revenue chances are that unit can only be considered a 1/2 unit, if two or more are in the same area then instead of considering the total revenue divided by total number of units, we are going to have to consider those half units)

These can be done manually by assigning a number to a column ie: 1,0.75,0.5.0.25,0 and summing the total over the total number of units

the following rough formula is accurate in recreating the result automatically so saving time

The problem lies with the "content" of the formula as the figures used would subject to more scrutiny than the resultant outcome which is fit for its purpose

Can I change this formula

=IF(OFFSET(F4,0,1)"is less than"3000,ROUNDUP((G4/3000),1),1)

in f4 to a function and replace formula (in f4) with "=function1"

Any help appreciated