I'm going to get a lot of criticism for this; but here goes.
I wouldn't use a head knife if you paid me!
Without a doubt the worst cutting (for me) and most dangerous tool made for leather workers.
I do everything by the KISS principle.
I keep it simple. Every leather project I've done has been cut out using scissors. Please keep in mind I don't make saddles so most of my leather has been the 8-9 oz weight. You saddle makers, I understand the need for something else. But for those of us who are working in the lighter weight leathers, there is no need to use a cannon to kill a cock roach.
Go to Tandy and test out their premium leather cutting scissors. They are not cheap, but they work like a charm. We've all been using scissors since kindergarden so you won't need lessons on how to use them. Keep them perpendicular to the leather and you won't have accidental undercuts. You may also want to pick up a good pair of work gloves to pad your hand, but other than that.... give it a try.
David Theobald