David,
I'm running the SEP 24/17/3. It gives me the exact performance I was looking for on the Raven which was a good balance of speed and vertical which I didn't have before.
On the PC-21 I would think you would want a little more speed; and since we have some data on the SEPs, you would probably be right in the ballpark with a 24/18 or 24/20, but I'm not sure of the sizes available. If you're looking for a Biela, then I'm sure the numbers may shift slightly due to the of composite vs. wood.
Let us know what you come up with.