alright, i stole this from a trailer towing forum, but itperfectly explains the "chalk test" for figuring out air pressure. its not scientific but it works. i used it on my jeep wrangler and ran 28psi for 2 years with no problems and the tires wore perfectly! give it a try dude!
1. Put normal operating load on your trailer. 1, 2, or 3 skis, 1 boat...whichever.
2. Inflate tires on both sides to max psi. I say the max psi because this is just a starting ground...you shouldnt need any more than that.
3. Park where you have an open area ahead of you.
4. Mark a thick line across both tires. Only one side is really necessary.
5. Pull forward enough for line on tire to mark the ground.
6. Observe chalk mark on ground, Mark should be of the full tread width. If too narrow, deflate. If it only shows the outside edges, you need to inflate more, BUT you shouldn't need to inflate if you are at the max pressure. You may need to go up in tire size or rating.
7. Deflate accordingly. If you think you need to take a lot out, start at 5 psi increments. Once you start to get close to the full tread, move down to 1 or 2 psi increments. Make sure you deflate both sides the same so the trailer is even!
8. Repeat Steps 4-7. (Remark, Drive, Observe, Deflate)
9. Once even tread contact is observed, Record that air pressure for future use. That pressure will provide the best traction, best wear and be less likely for blowouts.