Paul,
What I am proposing is to write a snippet that would render fields ‘a’ and ‘b’. The snippet would handle the calculation and update of field ‘c’. Field ‘c’ would replace your formula field—just change the UI Only field type to ‘Number’. I am pretty sure that the snippet will ‘fire’ even when backspace is used. The snippet can detect if field ‘a’ or ‘b’ is empty and then use a ‘0’ for field ‘a’ or ‘b’ in the calculation of ‘c’.
Thanks,
Bill
