I understand your reasoning that reducing the shot weight reduces resistance and therefore should reduce pressure and be safe.
That said you are considering one variable and while more likely than not correct for most powders it is not a lock for all loads that can be depended on.
I repeat that I did not find Armbrust stating you can safely interchange load data between lead and bismuth. I think in many cases it would turn out safe, but the variables of applying your course of action as a general rule is unsafe without taking into consideration all factors to include the gun it will be shot in.
I wish you well, but not everyone out there reading our posts have the knowledge and judgement where your course is safe or not.
Developing your own load is great, but before sharing it as valid and safe, it should be tested.
Lastly I repeat there is alot of tested bismuth data out there that can be used without going your way.
