also, something to consider. Different servings have different laying qualities. In example, if you use a power grip serving(my "go to" serving), it stays perfectly round even while under tension. Doesn't show much wear either. On the other hand, serving like halo, crown, and i'm sure i'm missing a couple others will tend to lay flat on a string.
I believe that's why some people believe that the halo's and crowns have a slighter release. I just prefer the power grip.
With them laying on the string different, different sizes will be different diameters. A .021 in xs .021 will be closer to a .018 in powergrip. a .030 in Halo is pretty close to a .025 in power grip.
Sorry for the detailed response, I just didn't want you to be investing money in a serving that isn't necessarily going to work out for you. If I used FF, I would be of more help.
God Bless,
Daniel