For muzzleloader casting roundballs, or any other soft lead projectile with pure lead, temperature is not important beyond hot enough to get the mold filled out completely.
Where temps become more important is with lead alloys such as wheel weights and linotype. A melt too hot with alloys causes tin and antimony to seperate from the lead. This is fixed by fluxing the pot. (Add a pea sized chunk of Beeswax to the melted surface and let it melt. It will smoke so light the vapors on fire with a match. Stir the pot and scrape the sides of the pot as the flame starts to diminish to release any trapped impurities to the surface. I use an old butter knife to stir and scrape.) With alloys you can maintain a correct temp and less fluxing without messing with a thermometer by just paying attention to how well your mold is casting. If you have to wait for more than a few seconds for your sprue to harden, and if your bullets are coming out frosty, you're too hot. Back your temps down just a little until your sprue hardens quickly, your bullets are nice and shiny, and they fill out completely with crisp sharp edges.
I cast outside only, so I'm at the mercy of ambient temps. Casting in the winter takes a different process of time and temp than casting in the summer. So I just let my molds and my bullets tell me whats right.
Now with pure lead, all that really doesn't matter. I run my pot as hot as it'll go and cast away. My pot will not get hot enough to frost a pure lead roundball, mainly because there's no tin or antimony present, which is what "frosts" in the other scenario.
If my roundball mold gets too hot, as in the sprue takes too long to harden, I have two choices. Slow down my casting rate, or, run another mold into the cadence. That's usually what I do and cast some 255 grain flat nose that my son runs in his Remington 1858 revolver. He likes pure lead and light charges in that.
I still flux the pure lead pot but it's to remove impurities that accumulate. I ladle pour and about half way through a pot the surface of the lead will get looking "dirty". This is from repeatedly dipping the ladle in the melted lead which intruduces oxygen and causes oxidation of the lead. The oxidation is lighter than the lead and some of it becomes trapped below the surface. Fluxing, stirring, and scraping will bring this "dirt" to the surface where I can skim it off with an old spoon.
I usually just cast one pot and I'm done. But, the fluxing/stirring/scraping point is a good point to add more lead and refill the pot if I wanted to cast more. The beginning of each pot needs to be fluxed and cleaned anyway so that's two birds with one stone.
