Easy. Jack the car up, put front and rear tyres on 2" high blocks. Measure bumper, both ends, to floor. Use another matching 2" block to measure to if you like to give same as sitting on ground would. Jack up front, remove 2" blocks, drop car back down and measure again. The difference each end is how much it will drop or raise as a result of 2" at front springs. Of course, you also have to measure compressed spring height to even have idea how much removed spring gives that much drop and how much travel will it still have as usually a softer rate than if made as a lower spring.
