Rey, this is what I've done with cast bullets for over 50 years & it's done pretty well for me with very little adjustment. I've posted this formula on several forums & sometimes you need to bump it up a point or down a point but it's seemed to be really dependable for me on game up through hogs, many elk, my bull moose & African plains game, beyond that I can't offer any advice because I have no experience on the megatrons.
Here's what I've always done as far as alloys but remember one thing about wheel weights, they've changed over the years & that's why you "might" need to juggle them up or down one point from what I post. Earlier WW's had a bit more antimony than later WW's, like 4% for the older & the later one's something like 2% so there's a slight difference making the old one's a bit more brittle.
So, here's my formula & I always start out at 1000 fps.

1000 fps I use 70% wheel weights & 30% pure lead
1100 fps I use 80% wheel weights & 20% pure lead
1200 fps I use 90% wheel weights & 10% pure lead

Above 1200 fps I just used straight WW alloy, air cooled.

Going the other way now.

900 fps I used 60% wheel weights & 40% pure lead
800 fps I used 50% wheel weights & 50% pure lead

This has always been really close to ideal to get good penetration on lower 48 big game, good accuracy, accuracy & zero to very minor leading with correct fitting in the throats.
As I said, you might have to juggle this up or down one point depending on your supply of wheel weights.
I've never felt the need to add expensive tin & in the photo's below are some of the animals taken including a few elk with the 41 & 44 magnum with full penetration, one was almost length ways with the 41 magnum & a 230 gr Keith bullet & it did exit.
The bear is a 6 1/2 ft bear taken with a 44 flat top OM with an exit (one of seven)
The bullet is my normal 70/30 alloy & was one of few recovered from a very large feral hog after pulverizing both front shoulders from a 480 or 475. Almost 1" expansion & only lost 2 grs!!














Dick