There is a theory of reduction, like in the link OP posted.

17 = 1+7 = 8

28 = 2+8 =10 = 1

And so on and so forth.

The other theory is that every number is counted separately.

But Q often uses simple gematria in his posts, meaning that:

1=A

2=B

3=C

...

17=Q

And vice versa.

So I would venture to guess that double-digit numbers are probably only usable up to 26, and after that, they'd have to be simplfied.

Does that mean that 2626 means ZZ or BFBF? I guess whichever one makes more sense.

But your guess is as good as mine. This is just how I see it.