What material should you get?
Whatever you want and with each option of material and the number of strands, etc, you're going to have to adjust the serving diameter.
You want to finish your serving diameter around .110 for a standard carbon arrow nock(GT, and a host of others).
IF you're wanting a combo that will work for you,
get b55 material. Order halo .019 serving.
12 strands of b55 and Halo .019 will get you very close.
If you're wanting fastflight material, it'll vary with each material. I have some of the, discontinued, bcy x material that I could let go for a decent price IF you don't mind what color. That is all I have built strings with for the past 5 years and I still have a good stock in it. I am sad that it is no longer an option. All good things come to an end, I suppose.
For 12 strands of X, you'll need power grip .032. For 18 strands of it, you'll need halo .019. or you'll need xs.021, and so on and so forth. Everytime you change material, strand count, or nocks, you'll have to readjust.