For storing calculations when using forms, I have found that if you write your formula in VB code instead of using the expression builder, that it stores the calculations in YOUR Tables! Example:
NAME
OF
FIELDS (in your table)
1stAmt
2ndAmt
3rdAmt
TotAmt
Next, make a form.
Then, Go to design mode and right click properties. Go to event.
Go down to On Enter.
Click and go into the code builder (not the expression builder). As soon as it opens up, you will see that the builder has automatically put two lines in there for you. That is your first line and last line of code. You insert your formula into the center. The formula for the above is:
TotAmt = ([1stAmt] + [2ndAmt] + [3rdAmt])
Your code layout will look like this:
Private Sub (name of field)_Enter
TotAmt = ([1stAmt] + [2ndAmt] + [3rdAmt])
End Sub
Hope this solves your calculation problems, It solved mine.