A popular way to deal with tire and chainring clearance issues with the chainstays is to use a yoke. In BikeCAD version 15.0, in addition to traditional chainstays or wishbone chainstays, we can now choose from a collection of commercially available yokes. Of course, when a yoke is used, we will not need to bend our stays as much but you still may want to apply a single bend for crank arm clearance. Once we've selected our desired yoke, we'll want to set a value for F which is the distance from the center of the BB to the front end of the yoke before the yoke has been machined to conform to the BB shell. If your objective is to machine as little off the yoke as possible, you can look at the side view of the yoke and set a large value for F and then reduce F until the top and bottom edges of the yoke just make contact with the BB shell. If you want a tighter rear end, you can reduce F even further and machine more off the front end of the yoke.
Dimensions X and Z define the attachment points for the chainstays with respect to the unmachined front end of the yoke. For plug style yokes such as this one from Paragon, we will not want to change these values. However, for yokes such as this one by Konga, the builder can tweak X and Z to define their own preferred attachment style.