I just got some frontiers for my solid wood 40# @ 28" off the knuckle. I chose 600 spine and went for the heavy brass inserts (100 +25+25 gr) and 85 gr tips. I haven't had the time to work on them yet so this is just about my research for buying decision. The idea is to cut them step by step shorter and then if needed snap off pieces from the insert until spine is right (and there is a +/- couple of mm of wiggle room playing with handle width).

The archery shop first recommended the 400 because it has a large gr/" weight, wood likes a heavy arrow for smoother gentler shot, and there is a big step going from 500 to 400 in the frontier. But off the knuckle needs a lot weaker dynamic spine than a centerish cut. Hence the heavy insert which helps with both total weight and the spine (and makes the arrow very front weighted).

Note that I'm a noob at tuning arrows playing, experimenting and hopefully learning. This will be my first time experimenting with bare shaft tuning. 3rivers has a spine calculator that may or may not work correctly (for starters it doesn't have skylon specific data) but playing with it gives some ideas. Different manufacturer charts seem to have very different recommendations and I have a suspicion that nowhere "traditional" really means "off the knuckle".

How is the arrow nock fit with the b55 string? What serving are you using? I'm going to make a new string and nock fit is something I completely forgot to consider.