How do I know how far I should trim up my outdrive when Im doing top speed runs in my baja outlaw?

If you have a GPS unit, watch the MPH. Trim up in short "bumps" and watch the speed. (give it a second to react) When it slows down, go back. With my Fountain being a stepped hull, I also play with the tabs. I get the most speed with the tabs just above neutral on my indicators, but they are about parallel with the water. Not sure if playing with tabs would do anything with a traditional hull. But it doesn't hurt to try.

you can also overtrim a little to get to max RPM then slowly bring the trim down to gain speed while keeping the RPMs up.

You really need a gps to do this right.

Remember the lag with a gps so make small adjustments slowly.

So is it better to push the throttles down, then trim out slowly. Or to overtrim, then bring it back in slowly. I'm testing props this weekend, so any help appreciated.

it really depends on your boat. if you don't have any trouble reaching top RPM then no need to use the overtrim method but it won't hurt to try both.