Front Limbers

I have started really taking front and back limbers seriously after rereading what Coach wrote on the tumbling progressions. Here is my question. When working on front limbers, should the heel remain flat on the ground or can it come off? I am guessing it should be flat because this is much harder.

Preferably flat but do what you gotta do. It's just harder to stand up while on the ball of the foot rather than the heel.

Thank you. I will try to keep the heel down then.