Loading subsonics is a little different than loading standard rounds. You want to start "High" and work down till you get to 1000 FPS.
If you start out low and work up, you may stick bullets in your barrel.
It has been my experience (with regards to 308 subsonic loads) that boat tailed bullets (of any weight) turn and fly base forwards out of barrels with twists of 1 turn in 10" when fired at subsonic speeds (they usually make the "turn" inside 50 yards).
Flat based bullets fly true at subsonic speeds (in my barrels rifled 1:10) and group well out to 100 yards.
My standard load for subsonics in my Remington model 700 SPS with 1:10 twist (20 inch barrel) is 8.8 grains of Trail Boss in Lake City brass with a 150 grain Sierra Flat nosed 30-30 bullet (Wolf Large Rifle Primer).
Velocity is right at 1000 FPS and the bullets are subsonic at 80 degrees F and at an Altitude of 200' asl.
Keep in mind that as the temperature gets closer to freezing the easier it is to break the speed of sound (this goes for altitude as well, the higher the altitude the thinner the air the easier it is to get your rounds to "Crack").
So, your "summertime" subsonics may break the speed of sound in the wintertime and your near sea level rounds may "Crack" at your cabin/shooting spot in the mountains.
Hodgdon lists the starting load with Trail Boss as 10.0 grains using a 150 grain bullet.
Start there and work down slowly and you shouldn't have any problems.