That's probably what I'd do.
Start at 100% fill and work backwards 0.5gr at a time until subsonic then back up 0.1gr to find max subsonic load.
There's a fair chance you'll find that 100% max load is still subsonic.
Still happily answering to the call-sign Peetza.
You do not HAVE a soul. You ARE a soul. You HAVE a body.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.