Same as a BII

you need to compress the springs all the way to get the length and you dont want to buy a shock that is 2inches longer for travle . thats why you measure it so you know the limit on the travle in the first place, if you max out your trucks travle why would you need a shouck that can go passed it

Ok well how do i fully compress the axle?

try a a car ramp on one side front tire the stock axle set up does not have very much travel i would say a car ramp is going to pritty much compress it as much as its going to go and getting a shock 2" longer wont really help that much the stock radius arms can only pivit so much you can do some work and make them flex pritty good but then you have other problem like the steering starts doing funny things when it flex's thats why if you wont some good flex people straight axle them but i have seen a couple of ttb axles that will flex great but it takes a lot of work to do so. here is a pic of what my steering did when i got my ttb to flex a little and trust me its not broken i drove it a hour and half back home that night.