Kelli wrote:
<<How do I "fill down" a formula in a column based not on an absolute row
range but on the last used row of a different column? I've tried every
syntax I can think of but can't make it work.

Basically, I want to fill down from "O1", with the last row being
equivalent
to the last used row in "A".

(Normally, you would have column labels (headers) in row 1, so you would
actually enter the formula in O2, then fill down from there.)
But, the simplest thing you can use would be something like the following
(untested):

Dim LastRow As Long

LastRow = ActiveSheet.UsedRange.Rows.Count 'assuming the data
starts in row 1.
Range("O1:O" & LastRow).Formula = "=A1+B1/C1" 'or whatever the
formula is.
--
Regards,
Bill Renaud