I use a caliper on the major and one of these and am usually pretty close:
http://www.amazon.com/Grip-Thread-Pi.../dp/B004ROIU2A
Most of what you're doing with the slambolt is threaded based on your call-out, so as long as that matches you are fine. The rear thread on the matrix is fairly coarse, so you probably have a little slop.
This thread has also been helpful to me in the past:
http://www.pbnation.com/showthread.php?t=803907