I'm not sure if you have a neck sizing die, but I use my collet sizing die to close the neck of a case enough to lightly grip the bullet. Then chamber it, carefully eject it, and measure using a Hornady comparator. When starting a new load I'll do this about 10 times. Not all of them will be the same, but usually 7 or 8 of them will be and it's worked great for me so far.