For any single type boat, which is the most effective and efficient heel position with respect to the waterline?
Your question about heel hight in relation to the waterline is very interesting.
The answer is about you, the sculler. How flexible are your ankles and your legs and hips?
Most scullers want to get the longest stroke possible. Therefore if they can achieve a tight compression at the catch and reach further forward this is good for their boat speed. If you raise your feet so your heels are higher above the water line, then you may be able to reach further forward.
Can you get your shins vertical at the catch? try raising or lowering your shoes and get someone to video you so that you can see where is optimal for your flexibility.
Let us know how you get on