Just go with medium , as has been pointed-out, as a starter.... while I totally understand your ` fears` / apprehensions [ we were all there] it`s easy to amplify possible issues/snags..there are so many things going through your mind....it can be daunting.
If it turns out to be a little lighter density than you`d like, you can tweak it on the next order...no-one will be any the wiser as you`ll look no` worse` [thinner] than you presently do.....if it`s a little too dense, you can easily thin it a little by dabbing on spots of hair-removal cream [ Nair] randomly to the knots. Don`t fret too much anyway.. just send in your pics. to John with your order and he`ll know what will work best for you....just get your template done reasonably accurately, and a decent hair sample to go with it [take hair from a few areas as the hair at the back/crown is always darker than towards to front of the head]
Good luck with it .