I could take some of my 405 grain bullets and chuck them in a fixture and drill out 55 grains from the rear easy enough. My fixture would, once I got the depth and diameter down, be fixed so I could always make exactly the same bullet.
DON'T do that, please.
You're new to inline muzzle loading. Your CVA is a 1:28 twist rate. The .45-70 Government cartridge with a 405 grain projectile was meant to shoot well from a 1:22 twist rate rifle. So your twist rate in the CVA should be close enough.
Pressure will be a function of the amount of powder used + the weight of the projectile vs. the diameter of the chamber. So IF you are worried about the pressure from a 405 grain projectile, then simply don't use 100 grains of powder.
You could get
Triple 7 pellets, in 30 grain size, and shoot two of those for a 60 grain load, which will be fine for deer with that 405 bullet. Your load is then 10 grains of powder under the load for the cartridge. No worries.
https://www.muzzle-loaders.com/hodgdon-triple-seven-30-grain-pellets-50-30-powder.html
BUT you already have the 50 grain pellets, so you should try shooting just ONE of those, and if they are accurate and your state does not specify a minimum powder load, you can use
one of those. Should still be fine for deer out to 100 yards.
IF you think that's too little powder, or you want the option of going past 100 yards, then the BEST idea would be to use the loose
Pyrodex or
Tripple 7 and a powder measure set at 70 grains and thus duplicate the .45-70 load. You would then use the 50 grain pellets for target shooting at the range. IF you simply cannot do without using the 50 grain pellets in the field, then do what folks used to do when the only pellets on the market were the 50 grain type, but shooters and hunters found their rifles were more accurate with a slightly higher amount of powder. You load a pellet, followed by 20 grains of loose
Pyrodex or
Triple 7, and again you have a 70 grain load.
Another option is to load a 50 grain pellet followed by a 30 grain pellet for an 80 grain load, which gives you the convenience of pre-measured pellets, but doesn't stress your rifle or your shoulder as a 100 grain load would.
My suggestion is to use the pellets for target shooting, and to use loose powder and a measure and to tailor your load for accuracy.
LD