Most of us have
lead hardness testers on the end of our thumbs. If you can mark it with your thumbnail, then it's likely soft enough. Try it on various lead samples. Buy some "99.9%" certified if you want a reference sample if you like. I do have a pal with a hardness tester, but I've never felt the need to take samples over for testing. I should as I'm going to give him all the alloyed stuff.
I'll mail you an alloyed and heat treated bullet made for high pressures if you cannot find "hard" lead for reference.
Once alloyed, it's alloyed. You can dilute the alloy by adding soft lead, if you have plenty of lead. But if you have plenty, just use the soft stuff. No simple "un-alloying" Pb as I understand it.
Heat the mould by keeping it near the pot, some dip a corner, and then by casting. Then it will reach full-temp as you cast. Eventually it gets too hot. You'll see. It's one of those things that becomes rather obvious when you actually put your paws to doing it.
Just set up and cast 50 or so JUST to see how it goes, re-melt every single one of them. Your next 50 will be MUCH better. Eye protection and ventilation aren't to be skimped on.
I hate messing with wheel weights, especially these days with all the junk metal they are using. I have pounds and pounds already cleaned, but I may never use them. THE BEST WAY to deal with wheel weights is to let other folks mess with 'em or buy the metal pre-cleaned. Soft lead still appear reasonable to me on the open market (Ebay), I don't think cleaning WW material is good use of my time. And yes I can get them for free.
ALSO, I try to catch most of MY target shots such that I have a stump or a backstop I can mine for soft-PRB-only lead. Should I ever run out of soft lead, I can mine mine own.