Lowering the seat foam

I want to replace my seat foam, because the foam rinse to the carpet. I like to sit low in the car and in the seat. With a new foam, i sit to high and the feeling is like to sit on a big ball.
I want to lower the middle section and need a tip. Should i shave it in the middle and put in a softer foam ( to keep the vinyl tight ), or should i shave the foam on the spring side?

Thanks for your help
Marco