Counting strides and leaning back works. The best thing is to jump perfect the 2nd best thing is falling behind the horse but the worst thing is jumping in front of your horse.
Sit back. Don't lean on your horses neck to get to your two point position, let your horses neck come up to your two point position. Does that make sense? Keep your horse equally balanced on their back legs the same as their front legs.
I find counting down strides works. It helps to give you the timing for when she will take off.
Try to come on a very small jump couple of times on a light seat position before and during the jump and try to feel when the horse is taking off. Do it couple of times until you get the right feeling.
Hope it helped !
What I have found that works is keeping a light seat when I'm on the flat and if it's a low jump keep the light seat and have the horse come to you and move with them as they go over.
Go at her pace. When she jumps you jump. As she takes off go into a 2 point not before she jumps. Hopefully this will help :)