Since most of us simply buy off the rack radius rods, we go with whatever length Speedway, Total Performance, Pete and Jakes, SoCal, etc carries. If possible, use the longest ones you can buy, something like 32 inches I think.
In some applications it is impossible to get the draglink and radius rod to be the same length because of how the steering mounts, but try at least to get the arc points to intersect, like the one in this picture.
Don