Hammockgear.com is great, I'd go with a 20 vs 40 at first.
I got a 40F and love it, sometimes combo it with a fleece liner to get close to 30F.
-Cphobes
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I just picked up the last 30 degree Leighlo TQ. Same setup as you with the 20 degree Pheonix UQ. Should be good for most 3 season camping around here (Canadian Rockies). I always carry a toque and compressable insulated sweater that I can throw on if we get a real cold snap.
Cheers,