Chris,
Use the grease cookie thickness and mix that works for you. Looks to me like I would not change a thing on the .450

As to your .500; it came to my mind to ask what your bullet lead hardness or mix is? Maybe softer or harder bullets would tighten things up?
Can't remember, are the barrels on the .500 Henry rifling or standard land & groove? Taper bore maybe? Could try a grease groove bullet that is at or +.002" of the groove-groove diameter just ahead of the throat? I have a .500 3" that is abysmal with paper patch but shoots great with harder grease groove bullets.
- Mike