i am using a form based on [invoice customer details] with a subform [invoice stock] for inputting stock from a list linked to [stock file] table.

once the stock is selected dlookup is used on update to complete the [retail], [cost] and [vat rate] fields in the [invoice stock] table.

the subform is used to add as many items as required to each invoice. a textbox with sum([retail] is used to calculate invoice totals.

i need to add another text box which will then be used to recalculate [retail] to equal a prefered sum.

for example the total invoice (sum([retail])) might be 198.90 but the user might want to round the invoice to total (sum([retail])) 200.00.

the calcutaions on retail would need to be percentages for example the 200.00 is a 0.553% increase on the origional sum, i need to make each [retail] increase or decrease by the desired percentage.

any suggestions???????????????