This is a common problem that I see a lot from users. You can't use that calculated value because it hasn't been calculated yet. So what you have to do is run your aggregate query without that calculation in it. Then take the results of that aggregate query and create a second query, and there you can do your subtraction. This is very common. In fact, I have another video on this coming out soon, so if you still can't figure it out, hang in there. See also Enter Parameter Value.

Don't put your calculated fields in an aggregate Query.
Build your aggregate Query then build a second Query based on the first Query and put your calculations in the second Query.

Guess Richard is faster at typing than I am.

I'm living 4 minutes in the future. :)

Thank you Richard and Kevin. I have now worked it out. Appreciate your help

QQ