Using a few straight lines -- down the center, across the stop length (where the bridge will sit), and across at the low point of the c-bout purfling -- and circles of 32- and 64-mm, I'm trying to make the f-holes look right.
I'm using the lay-out scheme featured on Michael Darnton's website, as well as an image of a Brothers Amati f-hole he likes. Even with all that, I still end up adjusting it a bit by eye. And in doing so, hope to train my eye in the long-run. Finally, say 'good enough' and cut the holes.
At this point they've got something of the shape, but still a little too thin. How to decide that? I make them wide enough to get the soundpost through. I'm sure there must be a better criterion, but that works for now.
After a bit of clean-up work, I think I have them ok.
The poster below is of a 1666 Nicolo Amati, son of one of the Brothers Amati, and what I used as a model for my eye. Again, not a perfect copy by any means, but something I hope works and looks decent.