When running a Roots type blower (BDS, Weiand, Blower Shop, 6-71, 8-71, 10-71, 12-71, etc,) and you have either a 1/2" pitch, an 8mm rib, or even a 14mm rib drive belt, the object of a blower belt not slipping isn't "tension" like a normal V or serpentine belts use because blower belts rely on the "teeth" of the belt to prevent slippage. What's hard on a blower, as well as a crank snout, the front main bearing, as well as the blower drive bearings, is a belt that is too tight. You'd be surprised how much a blower "grows" when it gets hot and all of that aluminum expands. It can take a belt that has an inch or so of play in it when it's cold and make it tight as a drum when it's hot, which as I just said, is really hard on things.
Some people say to adjust the belt so it has half a "twist" of "slop" in it when cold. My trick is to just run it loose, then let the engine warm-up and THEN adjust the belt after the engine is at full operating temperature and that blower, manifold, etc has expanded to it's running size, THEN adjust the belt so it has no slop, but isn't "tight". Wherever it ends up when it's cold is just where it's going to be. You aren't running the engine cold, or at least not hard when it's cold, so don't worry about it. Just warm it up and drive like normal once it's reached its running temperature. Doing it this way takes-out all of the guess work as to how loose the belt needs to be when cold, or how much the blower might, or might not expand once it's hot. Just do the adjustment when it's hot and take all of the guess work out and you'll be good to go. Remember, not tight, just enough tension so it isn't slopping around once it's hot. It ain't a V or serpentine belt.