you'll probably want to stay closer to 1020-1050 than the 1050-1100 range.
the closer you get to 1100 fps, the less atmospheric variation you'll have while the bullet stays subsonic. if you live in an area where it gets cold in the winter, and develope a 1100 fps load in the summer, it could quite easily go super on you in the dead of winter. i have a load that's 1053 fps avg that if the humidity is super high - like right after a rainstorm or when its foggy - i get supersonic cracks out of it.
as mentioned above - get a chrono. its almost mandatory for truly getting into the sub game. If you expect ANY kind of accuracy much beyond 50 yds finding a load in your gun with as low of an ES Possible is critical. Think single digits type ES. dont think it matters, try shooting a load with a 50fps ES at 50 yds. then repeat the same load at 200 yds. an inch group at 50 is going to look like a shotgun pattern at 200.
Besides that load data for someone else's firearm with chrono data may not work in yours.
I have two blackouts - a 9" pistol and a 16" rifle. i use the same sub load for both. that load has over 50 FPS variance between the two barrels. here's the shocker - the 9" barrel is the faster of the two.
regarding powders, 1680 is good, but tends to be fairly dirty. especially suppressed. DO63 is a lot cleaner, and runs about the same powder usage (i had to drop ~ .3 gr from 1680 data). 4227 should also give you some good results, along with a wider selection of powders. DO63 is commonly sold under the name Shooters World Blackout.
HTH