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.
Yeah, Even with the cinch tight you can't stand in one stirrup long without it slipping to the side. That is why we use the block, and yeah, if he steps away at the wrong moment and you can't swing right up, it'd slip. Any grownup would have that trouble, it's a lot of weight pulling it around, especially if he doesn't have a big wither. And the saddle slipping around with our weight in it probably doesn't feel good, so when it starts, they may move even more.
The good job for a helper, in addition to holding his head, would be to stand on the far side and hold the far stirrup down when you are getting up. The header helper can control three things at once, head from moving forward, and butt from swinging out, while pushing down firmly in the stirrup.