How about adding backspace to the class? (you know, the calculator does have backspace capability)

CASE nKeyCode = 127
* backspace
NODEFAULT
lSize = LEN(THIS.cSoFar)
THIS.cSoFar = substr(THIS.cSoFar,1,lSize-1)
THIS.SetValue()

And add nKeyCode 13 (for keypress Enter) in the INLIST() for default behavior?

By the way, you referred this class as txtCalculator but I found this classed as txtNumber (for those who are curious and want to check this class out).