I use to have that problem and it got solved by always selecting the parent table in the end, I think I put it in the form's refresh method, also assuming that:

-You have set up a parent child relationship in the DE
-You've specified InitialSelectedAlias in the DE (or is that just important in reports)

So basically if you select the child table to get a total, after doing your calculations you'd have to select the parent table again

sele child
calc sum(amount) to x for ... yadda, yadda, yada

sele Parent
Thisform.Refresh()

It worked form me anyway, HTH