Whenever discussion of how to design a new main- or whether the sailmaker knows his/her stuff- is undertaken people always quickly say the luff curve needs to match the mast's curve. I have asked this before but not sure I ever got a good response:
If the main matches the prebend when no downhaul is on, doesn't that sort of make the sail maximally depowered from the start?

I suppose the nuance is that if you make the sail's luff curve match that of the mast with the downhaul fully on then you might risk leach hook unless the downhaul is on significantly. We had a sail on an HT that had severe leach hook and it was a POS.
Of course this is likely really oversimplifying the question, but....
Thoughts?