Welcome to the forum.
Thinner air doesn't affect the speed of sound much until you get up near outer space. The main thing that affects sound speed is temperature. So you could come up with a load that is subsonic in hot summer conditions (speed of sound about 1160 fps in 100-degree dry air) but is subsonic in -40 degree dry air (speed of sound about 1004 fps). So your best bet for all conditions is to try to stay below 1000 fps.
Ballistics by the inch doesn't have statistically very rigorous data, but they did fire 147-grain Federal ammo and found it only gained about 100 fps between a 4" barrel and a 9” barrel. This is because the small amount of powder the 147 leaves room for in the case can't make enough gas to keep pressure high in the more distant parts of the barrel, so you don't gain a couple hundred fps or more with it in a 9" barrel vs a 4" barrel like you do with 90-grain bullets. This will vary some with powder choice, but I suspect that any load for a 147-grain bullet you find that is 900 fps or less from a 4" barrel will still be subsonic in your 9" tube at -40F and up. Hodgdon lists such loads for both powders on their load data site. Maximum loads of #2 all appear to be at about that velocity.
