Actually, the rule of thumb is down one size in length OR pitch

So if you're using a 13x7, a 12x7 or 13x6 would be the appropriate size.

That said, a 13x7 is the uppermost size for this engine. I run a 12x7 Graupner on my 55AX and the performance is outstanding, so going to a 12x6 should be great.