Mark a line on the end of the sight block that is centered with the hole, and do the same on the bbl. The line on the bbl should be long to allow the block to move forwards and backwards. If you have a free-float tube without the regular metal end cap that would hold a normal set of handgrips on- expect to have to move the sight towards the noisy end 7/32". You can break out a dial caliper and take lots of measurements and get close. To find out if you're lined up without shooting, remove the bolt and carrier. Then, take a fired ctg and make at least three wraps on it with electrical tape and GENTLY insert it into the chamber just until it's snug and air-tight. Now blow into the muzzle and hold your finger over the pther end of the gas tube to feel if there's any air comming out.
Also, check to see if the gas tube rotated 180 degrees while being installed. This would cause the gas tube port to be facing up instead of down towards the hole in the sight block.
