UDF Recalculation And Speed

I'm working on a financial reporting project that should be in Access but unfortunately it must be in Excel. Some of the formula are complex and I have a UDF to calculate these values. I added the line

Application.Volatile

to each UDF but when I change the current month in a dropdown box, the UDF's do not recalculate. The dropdown box sets a period number on one of the worksheets - this same value is passed to each UDF. I tried using this code in my dropdown box :



Sub DropDown4_Change()

Application.CalculateFull

End Sub
but the PC just hangs. I have hundreds (more likely thousands) of formula in the spreadsheet and the recalc is recalcing everything whereas I just want it to recalc the UDF's. I even changed all of the sumproduct formulae to array sum if formulae which sped things up - that is until I forced the full recalc on the drop down change event.

So my question is: is it possible to just recalc the UDF's on 3 worksheets when the user selects a different period in a dropdown box?

And a supplementary question : if {sum(if(...))} formula are faster than sumproduct formula, would a (well written) UDF perform faster than a {sum(if(...))} formula?