Tam,
The question is a good one. It makes folks think, that is, if they will.
You are and have come up with the right answer. Now, G4F has given a workaround to you for the same situation. It is the reverse of what you are thinking. You are thinking, reduce volume in the case and use 44Spl data. He is giving you the option of seating the bullet as a 44Mag and getting the ballistic results of the 44Spl.
Same same.
The thing you are espousing is what I have been trying to say all along about OAL. OAL doesn't really matter unless the bullets compared are the same length, making the seating depth the same. If you have a 240gr spire point bullet, the bullet OAL is going to be much different than a LSWC. If seated to the same OAL, more of the spire point is going to be in the case. If your load is at maximum, you could be in trouble.
I am speaking in generalities about the loads, not yours in particular.
If a 44mag is seated to 44Spl OAL, and the bullets are the same as the data for the shorter cartridge, guess what? You have a 44Spl load!
At any rate, yes, you can do what you are thinking.