FLEX IS ALWAYS MEASURED AT STANDARD HEIGHT! (Standard Height = 66" top to floor for senior)
Cutting your stick shorter will make the stick feel stiffer while adding height will make the stick feel whipper. As a general rule for every inch you remove or add, you can multiply that number of inches by 2.5 to find the new flex.
Example 1: adding a 2 inch extension to a standard height 77 flex stick: Since we are adding height we need to subtract flex points as the stick will feel whippier so 77 MINUS (2*2.5) = 72 FLEX
Example 2: cutting 2 inches off a standard height 77 flex stick: Since we are removing height we need to add flex points as the stick will feel stiffer so 77 PLUS (2*2.5) = 82 FLEX