What is the best throttle point to trim for level straight flight? Half throttle, usual cruise speed?

Yep, that sounds about right. Being that half throttle, give or take, up to about 3/4 throttle, depending on the plane, is my usual cruise speed.

You'll want to initially trim at your normal cruise speed. Then when you change speeds you will probably get by with a small change in elevator trim to compensate. When you get ready to land (with power) get slowed down by about the mid point on the downwind leg and retrim. This will leave you less fighting of the elevator during the balance of the approach. If you don't have power (dead stick) you usually don't have time to do much trimming.

I trim at 1/2 throttle or just a tad less