smallie42,
The release spray is worth the investment. Try it.
I don't know if there's a formula for determining the melting point of an alloy, but the post below is about results from using Rotometals' 281 bismuth/tin alloy (58% bismuth/42% tin):
https://www.bassresource.com/bass-fishing-forums/topic/162966-six-degrees-of-separation/?tab=comments#comment-1844299
I'm assuming that you're powder painting. If so, let me pass on a source that smalljaw generously recommended to me:
https://www.tjstackle.com/
I use both their fluid bed and powder with good results.
When I first made the transition to lead-free, the question arose, "How do I match a 1/8 oz. jig head with existing molds when my alloys weigh only 85% (88/12) or 62% (pewter) of lead's weight?"
My approach has been to match rod/reel/line to make a balanced presentation. I'll leave you with a pic of a pewter, hair and hackle football jig from a 1/4 oz. cavity:
I don't think much anymore about how POed I was initially about having to make the transition to lead-free.