I have three sizes of serving I use: .019, .021, and .024. The .019 is when I need to add a second layer to a skinny bowstring. The .021 is when I make my own bowstrings and just need a normal sized serving on a normal sized bowstring. The .024 is if the original serving is too small, as in your case. If I use two layers of serving, I put the .019 on the bottom and the .021 on the top, because the .021 is less likely to sink in between the loops of .019 than vice versa. While one spool of serving is probably a lifetime supply for me, and I still had plenty left on my original spool from the 1980’s, I bought a spool of Halo because it was touted as being so much better than anything else, and it probably is.