ASC should take the ASCII value of the character and check it
You can also change the case in advance so you don't need to do that.
strCharacters = "GAA1234"
strLetter = mid(UCase(strCharacters), 3)
if strLetter is greater than chr(90) then
strLetter = chr(65)
endif
The added UCASE should change the case to upper case so you ALWAYS get a character between 65 and 90.