Cliff,
No matter what height you build your bench it will be the wrong height for some operations. When standing we can flex our knees or tip toe to alter our height to compensate. A good adjustable chair/stool can help overcome some of this lost mobility when sitting.
I have one machine that I normally operate while sitting down. About 8 years ago I bought a utility chair/stool from Staples. It cost about US$70 at the time. It replaced a standard office chair that had been using. It has a gas spring lift that adjusts the seat height from 20 to 30 inches. It has become my favorite work stool as I can easily adjust it to suit my working conditions, and even change it a bit to relieve back tension caused by staying in one position too long. Most everything is adjustable on it. The adjustable foot rest is nice as it lets me get my feet off the cold floor in winter and the adjustable back lets me sit back and contemplate what the next operation is going to be. Sometimes these are listed as drafting stools.
Just mentioning this as I think what you are sitting on is almost as important as the bench height.
Gail in NM,USA