Arrighi based his letter c on the letter a. That wont do for us. We are used to a different shape for letter c.
Heres how Arrighi said the letters a and c should be written. He used the same method for both of them and the same grid as well. To me, the letter c looks lopsided.
On the left stands the letter c based on the letter a, just about to fall over. On the right is a letter c based on the letter o. It is properly balanced. Here is one way of making it.
1. Start the letter c with the outer path of the letter o. Make the outher path about one-eight narrower. The left side should be the same width as it was before.
2. Make a copy if the lower half of the outer path. Align the lowest point to the bottom of the counter.
3. Reduce the left side of the path horizontally by half.
4. Align the left side of the path to the left side of the counter. Reduce the right side horizontally by a third. This takes you more than halfway to an exit stroke.
You can end strokes in several ways. One is to cut them off with a straight line. That will work. If you use a curve, you have to decide what it should look like. For our letter c, I'd like curves.
5. Make a copy of the outer path. Flip it horizontally and reduce it by nine-tenths, maybe more. Put it next to the outer path, about one-third of the height of the letter.
6. Lift the counter by an eight of the width of the top curve. Then remove overlaps and path segments you dont need. Join the loose ends.
7. Make another copy of the outer path. Reduce the size to about one-sixth. Then reduce it horizontally by a third. Put it close to the top and a little to the right of the exit stroke below it. Shift the path ends until they make good curves to the small path at the end.
8. Remove the path segments you dont need. Join the loose ends, and you have a passable letter.
I didnt use a formula to design the letter c. I worked first and measured afterwards. But at the time I wasnt trying to explain anything. I was just having fun.
You can make the letter e from the letter c by turning the top into a bowl. Understanding how a broad-edge pen makes a shape like that is REALLY useful.
Arrighi says the letter t should have a short ascender, just high enough to distinguish it from the letter c. Thats a start. Use the crossbar from the letter f. You may have to shorten it. The letter t should be straighter than the letter c, and the bottom hook shorter.