I don't use a chair. I went to try out a Hermann Miller aerilon [?spelling] chair as this is what Harriet Hargrave uses and loves. Instead I saw what the worker bees at the office furniture places use and got them to lend me one over the weekend. I loved it, took the borrowed one back, and actually bought one.
What did I get? It is called a Swopper. It isn't even a chair but looks like an adjustable mushroom. It is almost like sitting on a good balance ball so that I have to sit correctly. I can change the height to whatever I am working on. No arms to get in the way. I have NEVER regretted the cost as it is so much better than any chair.
You can get them online too.
ali