HS-6 is great for full power .45 ACP loads, and is extremely dense, taking up very, VERY little space... You could easily double charge a case. Shown below is the charge of HS-6 I use (8.8 grains) in an unsized WCC Match case, and it is barely 1/3 the way full. It would be just a touch fuller with a sized case. This charge achieves 842 fps avg from my 4.4" barreled SIG P220 using 230 grain Speer Gold Dots.
I am partial to Midway's LoadMAP data, where they show a very wide range of bullets that were available at the time of printing (late 90's), and they show many different charges for a given bullet, and their corresponding velocity and pressure, all the way until the SAAMI max avg allowable 21,000 psi pressure is achieved. Using this data requires you to not deviate from listed OAL, and this is a hindrance for some bullets used (TC and HP styles) because at such OAL's, they may/probably won't chamber fully in many pistols... But this is not the case for nearly every std "GI" ogived 230 grain FMJ RN bullet. These type of bullets from WW, Rem, and even the plated Speer TMJ-RN bullets using a workable, max SAAMI OAL of 1.275", get a max charge listed between 9.7 and 9.9 grains of HS-6, with velocities listed of 940-965fps.
Again, if wanting to go for a worked up max using LoadMAP data, you must use the listed OAL, or maybe just a few thous under and NO MORE. Good luck to you, and I'd give the HS-6 it fair shot.