I use an Oregon clamp on that sets all the angles and depth. Only takes about 10 minutes/20" loop including mounting saw in vice and attaching the guide. Also had one of those cheap grinders. Used a couple times but it took longer than the filing guide. Never could see trying to free hand it as I can't see where it saves any time.
I have the Oregon clamp on guide too but it seems that the angle scale on the left and right side were off a small amount from one side to the other.After a few sharpenings the saw wanted to cut to one side.I bought a Granfers and solved the problem.