Flex is always measured at Standard Height for Senior sticks (66" from top to floor, standing straight against a wall). This means that for an 85 flex extended length stick, if you cut off the extra 3 inches the flex should feel the same as a standard height 85 flex stick.
General rule for calculating flex: Every inch you add to a stick will decrease the flex by 2.5 flex points. Removing an inch will increase the flex by the same amount. Here is a sample calculation:
Adding height: I'm buying an 85 flex extended height stick. Since the flex is measured at standard height (66") and the extended stick adds three inches (69") I have to decrease the flex by 3*2.5 = ~8 flex. Therefore my stick will feel like a 77 flex at full extended height.
Removing height: I'm buying an 85 flex standard height stick and I'm going to cut it down by 2 inches. Once I cut off the 2 inches, I have to add 5 points to my flex (2*2.5), meaning it will feel like a 90 flex at my new height.