BeforeAdd Calculation

Hi,
It's been a long week and my brain is way past mush! <img src='https://asprunner.com/forums/file.php?topicimage=1&fieldname=question&id=14181&image=1&table=forumtopics' class='bbc_emoticon' alt=':blink:' />
I'm trying to add some validation using the BeforeAdd function to add two values together and compare to a third, and produce an error message if it doesn't work.
I've got some working where it is comparing two vales e.g.
if dict("RecordType") = "Excess on Receipt" and dict("QtyAdvised") >= dict("QtyReceived") Then

BeforeAdd = False

message="<div class=message><<< " & "Qty Advised must be less than Qty Received >>></div>"

end if
In this example, I want to validate a third value dict("Quantity") and error if QtyAdvised-QtyReceived <> Quantity.
This is sure to be simple but see sentence one!
Any help gratefully received