Another Form Question : Purchase Statistics

I have added a variable to the purchase stats form, which shows the LCY vat amount, as this helps with the invoice authorization process. (We buy "local" products in different currencies).

I have bascially put

TotalVATAmountLCY := TotalPurchLineLCY."Amount Including VAT" - TotalPurchLineLCY.Amount;

Which works fine, but there are cases we change the VAT amount (by 1p etc) but this fails to update the field until we exit out of the form. The other fields on the form are updated when clicking into the header.
I guess it is something to do with the LCY "Amount Including VAT" not refreshing till we exit the form.

Any suggestions?

(I have tried running the pre calculated amount with the exchange currency, but this gives slightly different figures to the Unposted Purchases Report, which then causes differences in the balancing process!)