Need some help: trying to get better hole shot when pulling tube.
I have a 16' boat with a 90hp prop is 21" pitch. What pitch would I need to drop the top speed by 10mph?

How do you know that you do not have the correct prop for the operating range of your motor?

You are asking the wrong question. As to dropping top speed 10 mph, why would that bring any improvement for tubing? I can put a prop on your boat with excessive pitch that will result in a 10 mph drop in top speed - along with the boat wallowing to barely get on plane. That gives you a 10 mph drop in top speed, but certainly won't help pull a tuber.

What you SHOULD say is "I want a prop with a very strong holeshot for tubing without concern for what top speed I get from the prop."

If you don't have a tachometer, you need one. You also need to know the recommended top RPM range for your outboard. This will be the RPM range that generates the most horsepower, but won't hurt your engine, provided in the owners manual. For a given use configuration of the boat, and the prop that's on the boat, you never want to exceed the recommended RPM when at max throttle. Over-rev is not good; at some point the rev-limiter will kick in, but ideally you never want to get to that.

The way to do this: "I have engine xxx with an RPM range of yyyy-zzzz for max recommended RPM from the manual. I currently have a 21" pitch prop turning XXXX rpm. I want to get a strong hole-shot for tubing. Should I try another pitch prop, and if so, what should I try?"

Fill in the xxxx yyyy and XXXX info above and then ask your question again. At that point, someone will be able to tell you if there is a prop change that could help you, or if you are just "a bit shy on engine".