Good luck, most of the load data you find online will be for a 16" barrel. I had to adjust the load up considerably for my 8.5" barrel. Subsonic data for a 208 grain A-Max was 8.2 grains for Lil Gun and 9.2 grains for W296. I ended up with 9.5 grains of Lil Gun and 10.0 grains of W296 without cracking the sound barrier. For what it is worth, I went with the W296 load because it was quieter and just felt like it had a bit more pressure to it. Pressure you will find out is probably one of the more important issues, just to get the rifle to cycle reliably.
I'll be interested in what you develop for the Hornady 225 as I have about a thousand of those. For now I am still working on the 208 grain A-Max and Nosler 220 grain HPBT. I picked up lots of the 208 grain A-Max bullets when Hornady had their big discovery about melting plastic tips and starting discounting the price of the A-Max. I also picked up 500 of the Berrys 220 grain plated spitzer since they were cheap.
One of my neighbors has a silencer for his .300 Blackout and he is getting near Hollywood quiet out of it with his sub loads, much quieter than my pellet rifle. At the moment I like my subs without a silencer, but it is an intriguing idea for hunting.