Adding shock

I’m looking at adding shock absorbers to my 34 ft lifesyle and I have a couple of questions, I won’t be using a kit , but doing the job myself, how do you determine the correct length of shock, I know it has to have room for the up and down movement. Do you first attach the shock brackets in the best position, I know the more straight up and down the better, then  do you measure the distance between the two brackets with the weight off the wheels or with the weight down on the wheels , and do you buy shocks that fit those measurements that are retracted or extended, that is the issue I need help with, thanks for any help .

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