I'm thinking you'd need to start with 4.5" squares to end up with 12.5" quartered and re-sewn...no...that's not right. Let me just "think" out loud here, it helps me "compute" lol
4.5x3-1=12.5" nine patch, cutting and resewing gives 12"...
5" will give 13.5"...
4 3/4"?...nope, that will give you 12 3/4"...
Unless you either...
a) make your squares at a ridiculously odd measurements or,
b) make some squares a different measurement, or
c)go with a measurement that gives you a bigger final block and trim down to the right size...
You won't get a 12.5" final D9P block.