If you're talking about the ones on mandrelbends.com, they give the measure of the centerline radius. Don't hold me to this, but I would think you would measure the distance between the middle of the parallel pipes, and then take half of that.
On a 180* fitting, I believe CLR would be the centerline of the pipe leg to the pivot point. Which means measuring from the topside and measuring the centerline of one leg to the centerline of the other leg and dividing by 2 since the pivot point would be exactly halfway on a 180* bend.
edit: what mxhunter147 said ^^^^ I took to long to find a picture.
Three other methods of measuring;
1) Measure outside to outside of pipe, subtract the diameter of the pipe, divide that number by two, thats your CLR ie: 14" outside to outside, subtract the 4" pipe diameter, is 10", divide by two, 5" radius
2)Inside to inside, add the pipe diameter, divide by two