2 subforms - calculate from a field from each = main form answer?

[Carl Harris]

I've looked through various forums and just can't get
this.

I have two subforms

a) ordersbydate (orders placed in date range) - record
source is a query called "ordersbydate"
b) DMqtybydate (direct mail that has gone out in date
range) - record source is a query called "DMmailnosbydate"

these queries are both updated by a date range from an
ActiveX calendar command on a different form that calls
my form with the two subforms. (Hope that makes sense)

on subform "a" I have a field (field A) that shows no of
orders (=DCount("[OrderAmount]","ordersbydate") within
that date range.

on subform "b" I have a field (field B) that shows no of
mail items sent through mailshots (=Sum([DMail.DMqty])
within that date range.


My question is this:

The two subforms are obviously in another form
called "dataresponse". On this main form I want to put a
field that automatically shows direct mail response level
%. In other words - Field A (e.g 4 responses) as a
precentage of Field B (200 letters sent out) - should
equal 2% response rate, etc etc

I just can't work out how to target the two fields off
the two subforms and run the percentage calculation!!

Hope someone can help.

Thanks

Carl