Bubbagates or ?...

I'm mildly disappointed in the RPM of my new DA and also the EVO 45. I want to change props down one size to increase RPM and thereby hopefully a little bit of thrust. Because these propellers are so expensive I don't have the luxury of buying 4 or 5 and just trying them.

Here's my situation:

DA50= NX22x8. If I want to drop one size do I go 21x8 or 21x10? Which is just one step down?

EVO45= NX21x8. Is it 20x8 or 20x10 that is just one step down.

Thanks for your opinion.

Barry