Dan,

I live in Becket, and I sometimes teach at Heartwood as well. Not sure if we've met. Let me know if you have any questions or need a hand.

At 12' wide it's a pretty small building, so I suspect that you're kneewall height is OK. Especially if Jack OKs it. 30" braces would work fine (42 7/16 length) as Jim mentioned.

Another consideration, that I can't remember whether Jack addresses, is that you'll need some kind of girt or nailer to at the level of the joists to catch the end of your flooring (assuming there is a loft space). That adds even more joinery at the post/TB location. There are few ways to improve that situation.