I'm afraid I'm not going to be much help, but I'll contribute a couple thoughts anyhow.
The height depends on the configuration of your chair adze. If you prefer to swing it from the shoulder and elbow instead of from the elbow and wrist, or if its handle is longer, then you will want a lower platform. I tried Tim Manney's adze and watched him and a few others use it, and a lower bench height seemed to work better. You're going to need to experiment, but if that works for you, then the height of your saw bench or even your shave horse would be more effective. In that case, you may get away with drilling a few dog holes and using dogs and holdfasts to immobilize the chair seat. You may also find that the mostly horizontal force of the adze requires you to increase its mass, or strengthen the legs, or even increase their splay.
I'm embarrassed to admit that I'd forgotten to consider the adze, so after reading your comments I went back to toy around with it again. I agree with your comments about the length of the haft - had to choke up a lot on the Barr.
I've now lowered it to 30 1/2", and think that may work, because I'll not use the adze extensively unless I do more seats with sassafras or the like.
I have three benches in my shop. One ,the main assembly bench has a wide two screw vise for holding chair seats for edge carving.This one is at waist height. I have a smaller(2'x3') bench for assembly of upper parts that is only about 18" tall. The third bench is higher and is for carving. It is also small 2'x2'. When using the adze I use this bench with a fixture that holds the seat blank at an angle,partially upright. If this is unclear I can post pictures.
My vise consists of of a pair of Lake Erie tool works wooden screws and a 2"x4"maple jaw. The distance between screws is 2'(most seat blanks are less).The back jaw is the side of the bench. I mortised the "nuts" into the underside of the bench. I put garters in the outside jaw to hold the screws against the jaw.
That's an impressive setup. I went to the Lake Erie site to see the stuff you're using, and it is outstanding.
The reason that I addressed this question to you primarily is that, over time, I've seen and read a number of posts from you which describe your chairs, your processes and your shop.
Thank you very much for providing insights into the process of building these chairs.
are amazingly strong. Zero play, tight as a tuning fork, after a couple years hard use. I really don’t know how such lightness and strength is possible.
Wiley, I've read your comments to that effect before, and they intensified my interest in Jack's chairs, so I'm glad you posted them.
I won't attempt to minimize weight as a primary goal, but I really am enthralled with the tuning fork aspect of the chair's upper section, and I'll admit that I really like slender spindles.