First, have you ground the face of the rockers smooth? This is where the rocker hits the valve stem. If there is a ridge from wear, you won't get the valves adjusted right.
To adjust the valves on a new engine, simply follow the procedure for a warm engine. Last I looked there was a tech tip or two on it. It won't hurt to use the gap for a warm engine, as long as you're not running a huge cam.
After your 20 minute cam break-in, then reset the valves while the engine is still warm.
Another tip on a fresh engine is to use straight water for coolant. Seems like when I use antifreeze I end up with a leak, and straight water is a lot easier to deal with. Of course, add antifreeze once you know there aren't any leaks.
