I tighten it a bunch of times, gradually. The first time I tighten it just so the girth is resting on him with very little air between him and the girth, nothing constricting. Then, after I've bridled, I'll bring it up another hole so it's a little more fitting but still not completely there. Then I'll tighten it most of the way/all the way when I enter the arena, and I'll check it right before I get on to make sure. I find it is easier on them to tighten gradually like this and makes the tightness more 'accurate' as it lessens the effect of bloating. If I'm jumping, I'll have my trainer check it again before I go over anything. When it's fully tightened, I want to be able to fit 2-3 fingers between him and the girth snugly on the bottom of his barrel, but it shouldn't be a valiant struggle to get them in there.