NOPR

i need to replace my brake booster. the little rubber valve seal thing that goes around the metal part of the brake booster that pushes the master cylinder is dead, and air is coming out of it when i push the brakes. in my haynes manual, it says to replace the brake booster i have to measure the clearance between the brake booster and the the master cylinder with an hg vacuum and a micrometer and some crazy ****. my question is: how important is this? is there a way around it? and if it absolutely has to be done, where can i get it done and what should i expect to pay?