It depends on what you're going for, these are jumpers, if you're a hunter, you won't want to let that lower leg slide back, and unless you're jumping really large, you don't need it to.
You're right, if you have your leg properly at the girth, you're going to topple backward, this is because you're pitched way too far forward and aren't balanced. You need to work on the flat to improve your strength of your base.
The jumps aren't that large so you really just need to get up out of the tack and a little out of his way, not throw yourself up his neck. To get the feeling, practice 2 point at the walk and then at the trot once you become secure. This will build your strength as well as muscle memory and keep you from over jumping!