Hi,
Why not keep a copy of your formula tucked away in a named range, say "MyFormula" somewhere above your data, then in your userform include some code to copy and paste it to the appropriate place. e.g.
Range("MyFormula").Copy Destination:= Sheet1.Range("E" & Rows.Count).End(xlUp).Offset(1,0)
If you're handling several additional rows at a time, to avoid jumping backwards and forwards between the VBE and the Excel Application each time you enter new info, which can slow things down, consider only copying the formula in one block to a range of rows at the end of your process of adding new info.
HTH
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