>I am using buffering, but in this app it is possible that the user will change the value in the field several times. As I understand it, OLDVAL() returns the value that you got when you started the screen. This means recalculation would fire every time the current value in the textbox was compared to oldval() (assuming the user had made a change). When exactly does the control update the underlying field or variable???

I've had couple of forms like that, and I've solved it by having all the controls supplied with the .OldVal property, which was filled with .value upon navigation or just blanked on a new record. I used that just to keep the old value, in case user wanted to revert, or if I wanted to do incremental calculations of totals (something like Replace grand.total with grand.total+this.value-this.oldvalue).

Another solution would be to have an unbound control, which then behaves almost like a memory variable (just assign it the value of the field when you get to a new record or just before editing), and the value in the field will remain untouched until you update it in code.