I posted this as a question a couple days ago but have reached my own answer.  

First caveat!  Take this information and use it at YOUR OWN RISK!.  I have been called a moron by a lot of people dumber than anyone here.  Your Mileage may vary.  Taxes, license plates and dealer prep are extra.  Should you develop an erection lasting longer than four hours seek immediate medical attention.

First off my calculations.
https://i.imgur.com/UZotIxm.jpg

I started by looking at the 454 Casull from shootersreference.com which I think somewhat close to the 450 BM as far as a pistol length barrel goes.  From the top, determining gain in feet per second per grain of powder in the cartridges range.  Change in velocity versus change in charge weight.
54.8 feet per second per grain of powder in THIS CARTRIDGES RANGE.  I would have to lessen the load by 4.98 grains to go subsonic with the other things remaining the same.