Question on adding coolant to radiator

Yup I added the premixed 50/50 jug. Is it better to run the engine for a bit to normal temp then check the radiator after it cools for a bit? Just didn't want to pull the cap when its hot and have all the coolant shoot out.

But would that be the best way to tell how much coolant is in there or should it always be filled with coolant just below the cap area even when its cold?

Also the cap would need to be changed if I get a tank right?

Most caps now are designed for an overflow bottle. Your right, it's best to remove the cap when the engine has cooled down. You don't want a steam or hot antifreeze burn.
The coolant should cover the rad tubes when cold.