Valve lash.. Hot /cold ? How hot is hot?

After reading many threads about valve lash, I learned that the valve lash on the cam card is for hot lash. I also learned that many opposing opinions are held as to whether the lash increases or decreases with the engine warming up. Most seem to indicate that you need to tighten the lash when cold as it will loosen as the iron block and aluminum head expand with heat. Most would say for a hot lash of .016 inch to set it at .010 inch cold and recheck when hot. My cam card calls for .016 intake and .018 exhaust. So I am going to set it at .010 and .012 cold and warm it up. The question is.... how hot do I have to get the motor before I can check the "hot" lash????? I have an iron block with aluminum heads and solid flat tappet lifters.

The coolant temps are going to be lower than the block temps. What gauge do I use for the warmup? Do I run it for 5 minutes,, 10 minutes?? Or do I run it until coolant temps reach XX temp?? I have to at least be able to touch the motor and Can't have it steaming hot..

Any and all advice welcomed. Thanks