Setvalue Equivilant

I'm trying to create a database that we can use to track Vacation, Sick, and Personal days. It's a very simple setup, for each option I have a start value a place to enter the amount used and and expression field that calculates the time remaining.
My query calculates the difference (Expr1: timelosstbl!SickBeg-Timelosstbl!SickAccUsed)
but this value isn't stored, since it runs everytime the query is run. So I have another field that (SickTotal). I have a button that on click it runs a macro and I use the SetValue action and it takes the value in the expr1: field and copies it to the SickTotal field. I have another button that on click runs basically the same macro that copies the value in the SickTotal field to the SickBeg field. My problem is that when I click the button everyting works but I doesn't remove the figure that was manually entered in the SickAccUsed field. So what I thought I needed to do was instead of using a macro to run the procedure I'd do it as an event procedure but I don't know how to recreate the macro because when I do either me. or docmd. setvalue isn't an option.
So basically what I need is
A=starting figure, B=manually enter hours used, C=remaining time, D=stored value for C

A - B = C, C = D on click make A = D and clear B

Confused?

Thanks,
Rick