22 isn't bad. I aim for 22 - 25 and mine has been holding almost dead on 23 for a while. I have a remote thermometer in there so get the temp displayed on my Atomic Clock above my monitor. Grow room temp on the right and man-cave temp on the left.
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You still have your light on 50%? When those digital ballasts have actual watt settings like 400-600-1000 then you should have a bulb that matches the wattage as it's hard on the larger bulbs to be run at the lower settings I've heard. Also if it has a boost setting it's hard on the bulb to use that too. My 1000W Hortilux HPS conversion bulb cost me $170 and I'm treating it like a baby. It is actually only 940W but is designed to work in my 1000W MH magnetic ballast which has no alternative settings.
I'd think about getting a small T-5 unit or some CFLs for getting seedlings and small plants going. Even a shop light works great for small stuff and saves a lot of power. 23¢/KWH here so it's almost $90/mth just to run my 1000W 12hrs/day. Have a 400W going in the spare bedroom 18hrs/day so there's another chunk of change that's hard to come up with.
A Temp/RH controller would work great for you to keep the temps up a bit and only run your fan when needed.