This largely depends on your individual flying style. I typically trim models for level flight at a throttle setting of about half. Pilots that own "underpowered" models or enjoy the thrill of speed may trim at 3/4 throttle. Likewise, pilots attempting to maintain a scale appearance may trim as low as 1/4 throttle. What most modelers don't understand is that your elevator trim is essentially setting your airspeed, not altitude. A stable climb or descent can easily be achieved with a higher or lower throttle setting respectively. This technique will allow the aircraft to maintain a constant airspeed, so find a comfortable spot and trim to maintain it.