I take a fired, unsized case and squeeze the end of the neck with a pair of plyers just enough to keep a bullet from falling out. I then insert a bullet into the case about 1/8" or so and chamber the dummy round in my rifle. I extract it very carefully and measure the OAL. I then do this same thing with another 10 or 12 bullets from the same box and after measuring them all, I calculate the average OAL to the lands. From this number I usually start my loading by backing off another .010 to .030". This works very well with plastic tipped bullets as they are quite consistent in length. Works well with Barnes bullet too, though I seat these .050" off the lands. With lead tipped bullets like the Nosler Partition or Hornady Interlock I just load them to book max OAL.