29er on 2.25tires at 140lbs, I think the correct pressure is somewhere around 20psi, probably a hair under. For reference, I ride 26x2.35 at 160lbs, and I run 18/22 tubeless. You're lighter with about the tire volume/contact patch.
Here's how to tune tire pressure. Find a good section of trail about 10min long. You want a mix of the type of riding you normally do. Some technical, some not. Having a good tire gauge is key.
Start at about 25psi, run a session, drop the pressure ~2psi, repeat. Initially, you'll bounce off rocks and traction suffers. Then as the pressure gets too low you'll get pinch flats and it'll feel sluggish. Once that happens stop and add a few psi, and you should be good.