I center my mast after it has be restepped. Rather than the tape measure, I use my main halyard and a fish scale. Top of the fish scale goes to the halyard shackle. Length of halyard is adjusted so it takes about 15 lbs tension for the hook of the scale to touch the toe rail. I adjust the uppers so the tension to touch is the same on both sides. Adjust lowers to straighten the mast, then recheck centering.
Shroud tension at this point is just hand tight on the turnbuckles. When I get out on the water and get some heel on the boat, I tighten the leeward shrouds evenly from side to side (changing tacks) until the leeward shrouds are just coming loose at about 20 degrees heel.