I cannot, however, understand why a UDF cannot find a value on another worksheet on which to base a calculation to obtain its return value? To me it seems an unreasonable restriction, after all my public UDF "PrevSheet(cell ref)" does just that. I have tried running PrevSheet() from within VBA, which I thought was legal, but I get a compile error “Sub or Variable not defined”.
I did not know this was not representative of your actual file or I would not have invested the time in a redesign.

I don't recall this particular problem and a UDF should definitely be able to do this. What exactly are you doing to call PrevSheet that is giving you an error?