If you think it is tight already, I would be reluctant to go "too-tight". Do you tighten it a couple times, once when you saddle him up, and then once when you've walked him around a little? They do usually need a second pull. The other option that can ~help~ hold it in place ( a little) is a breast collar. I ride with mine moderately looser, when I have a breastcollar on.
Yeah, I'm compulsive about it. I tighten to the sixth hole where it is hard but I can get 3 fingers under the cinch. I put the left stirrup over the horn to remind me and then bring it up another notch before mounting. I test with my finder again and it's hard to even get 2 under. I pull on it with my weight to make sure it's going to stay snug. It's synthetic and probabably with my weight I was in the left stirrup too long. I don't remember it giving but at that time I was just working on going over.