Disclaimer: I'm just an amateur, so take my words with a lot of caution :-), And I just know about Mini sails.

Looking at pictures of land Yacht sails (especially R&J and my Plume Frog sail), the order of magnitude seems to be around 2-3% of draft for the seam shaping (not a lot).

Then you have the "extra draft" related to the "extra" Luff Curve, which is much controlled by sheeting (pull more => more curve in the mast => less overall draft.

Maybe however you are looking for the " flying shape" draft. I had Hijacked some pictures of a Sailworks miniSkeeter. Flying draft was around 9-10%

JMF