Reading Plugs and Prop Size

Intolerant1 asked a question about reading plugs and how long to get the color you need. That was a great question. I need to do the same thing soon.

To take that question a little farther, do you need to be propped right in order to get the right reading. Or if I'm under propped like I'm now can I get close with jetting then prop. I think I need to go up in size at least 3-4 inches in pitch to get close.

I would hate to burn up an engine or two testing props. This is kind of the chicken before the egg kind of question.

Thanks for the advice.

Jeff