If you want to run a motor at a set RPM at WOT and your current prop achieves this goal at 3/4 throttle what should you next move be? Will moving up a pitch (say 24 to 26) get you closer to your goal?

Yes, moving up in pitch will get you closer to your goal.

Is there a rule of thumb on how much a pitch size increase will change RPM at WOT.

The "rules" vary depending on hull, engine, water conditions, etc.
What are you running?

27 excalibur with notched transom, bravo 1 ratio 1.5 with propshaft 4'' from keel, carbed 509 with approx. 525 hp, Bravo 24 4 blade 5200rpm at 3/4 throttle.

I'll bet a 26p Bravo/1 will just about get you there.
A 28p might be a bit too much load on the engine.
Get yourself close, then consider labbing for the fine tuning.
Julie & ThrottleUp has a prop testing program and probably accepts trades.

PS: I'm assuming you don't want to exceed 5500-5800rpm.

26 was what I was thinking too. Looking to spin around 5200-5400 tops.

You could probably lab your existing 24p to reach your 5400rpm objective.

I guess its time to make a call...

Be sure to give her plenty of detail and a good set of numbers to analyze ( RPMs, MPH, etc.)
You won't make a mistake following her suggestions.
Good Luck .....