Ok, I’m confused, is the blank actually used in the middle of large column labels? Like is “A A” a number? how about "A ", " A ", or even " "? (All are three characters) If not then it seems like the base is 26, so I’m not seeing where the 27 multiplier is coming from, except that it’s also a case where there’s no 0 (like music?) and that might throw things off
Edit: Ok, the comment by Morti explains everything. It’s not the blank, it’s the A ≠ AA so there are more numbers before a digit is added thing.
Easy. Just be 64-bit. (Or unsigned, but that’s not as good.)