Cap'n Pete gives advice all through the book on measurements, oar type and length, thwart location, and so forth. I don't recall there being a "formula" or even mention of things like "forearm length." He's pretty much clear that leathers should be stitched on and never tacked, and even laments at one point that a herringbone or "baseball" stitch may be a lost art since kids don't make their own baseballs anymore! He says that leathers should be 12 to 14 inches long.
Here are a few paragraphs that may prove useful:
"The height of the thwart is most important, and in craft from 12 to 18 feet in length sometimes cannot be perfect, due to limitations of the boat. I've set thwarts as low as 7 inches from the bottom in very shoal, low-sided craft; this is not the best practice, but, if you have room to stretch your legs nearly straight out, it is quite workable. The height from the thwart to the rail runs 6 inches, more or less, in most boats--when more height is needed, oarlock blocks will work. I find thwarts located at 10 onches from the bottom and 7 inches from the rail a fairly happy medium, though mostly I don't quite make it, due to the design of the boat, so fudge out in various ways.
The standard thwart width when I learned the trade was 8 inches for flat-bottom skiffs, and the oarlock sockets were set 12 inches from the after edge of the thwart. This is a norm to work from, but is by no means ideal. Many fast working yawls and gigs of the past had thwarts only 6 1/2 to 7 inches wide, and I think this has a lot of merit--there is less wood to step over and bark your shins on in getting about in the boat. I also prefer a thwart that is narrow enough so you can hook your butt slightly over the forward edge. Even though there might be a good stretcher for your feet, this hooking over of your butt gives you a firm grip on things, especially when rowing in a seaway, even a slight one. It's quite apparent that the oldtimers who used these craft as a way of life knew what they were doing.
I find that many boats allow the grips of the oars to be too low, almost in your lap, which is awkward in many ways, poor application of your power, and just plain unhandy in any sort o a sea. I like my grips to come just about under my rib cage; if I am in doubt when building a new boat or converting an old one, I simulate a thwart, using a box or some blocking, fake up some oarlock blocks, and sit in her with an oar or even a long stick, taking into account where the water will be and fussing around with the set-up until I think it will work. It may never be perfect, but usually, using this method, I arrive at about the best you can do with a given boat. I'm sure that a person's build has a lot to do with thwart location; a person who is thick in the lower regions is going to crowd things more than the string-bean type. A very short person may not have much reach in his arms, let alone legs, and a really long person always has trouble finding a place for his lower ends. All this has to be taken into consideration--even the stroke the oarsman favors, though this can be altered if you can convince him that a change will make the boat go better. Some people do have bad rowing habits, which they often developed from using poor boats; bad habits can be eliminated, but it's often not easy.
The rough rules of thumb for placing locks and thwarts that I have given are just that--rough guides, somewhere to start. You can work from these, and, with a little trial and common sense, you can make the boat go pretty well and feel right. If, after some use, things don't seem right, try to figure out what's wrong and make the necessary alterations."
Not exactly a pat formula, but a good place to start.
To row is human, to erg divine.