That's another balancing act. If you are leaving lets say 4mm unused on your var. You have to find out if the rear pulley is completely open. If it is then you need a 2mm longer belt. If its not then cutting the boss 4mm will get you to the top. The trouble there is when the var opens the belt may not get all the way down to the boss reducing take off. A proper match of belt size, and boss length make it all work together. I had to add shims to the boss to get the belt down to the boss, and use a longer belt to get it all correct.
First find out exactly what your set up is doing. The Sharpie marker on the pulleys works best. Draw lines from bottom to top, front pulley, and rear. Then ride. That will show you the travel you have now.
Post up the results, and I'll help you figure it out. This kind of tunning helps to dial in the engine better as there are no variables in the cvt once you get that done.