Side wheels for your AO knob in field target are marked with yardages. Essentially when you get the 20 yard target in focus you put a mark on your side wheel. Then, in the future, when you don't have the yardages and you focus on the target you look to your left at the wheel and read the mark indicating the distance.
Side wheels come in many sizes of round as well as comma styles. Small wheels are often not very useful at distances between 40 and 55 because the focus change becomes very small at that distance. A large wheel alleviate's this problem because a small movement of the AO produces a large amount of movement on a large wheel. The down side is that large wheels are prone to put stress on your scope. A "Comma" wheel may be the best of both worlds as the diameter for distance is small where it needs to be, from 10-30 and then starts increasing from 30-40 and from 40-55 it's large.
One method of figuring out what is best for you is to get a piece of rigid foam from your big box store and start playing around with small, large and comma wheels. The foam is very easy to cut and will stay put relatively well on your side AO, enough for you to play around and see if ordering an expensive one is worth it.