On a GI type barrel just move the block forward the same amount as the thickness as the front handguard retaining sheet metal.  To center the gas block on a GI type barrel do the following.
1. Get your barrel installed with the barrel nut in the proper position.
2. Install the gas tube in the LaRue gas block.
3. Slide the gas block onto the barrel.
4. Line up the gas block with the barrel pin.  Tighten one set screw.
5. Do the bolt drop test.  That is remove your bolt from the carrier and with the ejection port cover open, drop the bolt carrier in the barrel.  If the bolt carrier drops right in you got it straight.
6. Red Loctite the other screw.  Remove the first screw and red Loctite it.  Done

On a match barrel there will be a centering mark on the bottom of the barrel.  Line up the cap screw hole with that mark and you are golden.  If the barrel only has one centering mark line up on the mark and install the front set screw and then red locktite the rear set screw.  Remove the front set screw and drll a dimple in the barrel around 1/16" deep.  Clean out the thread and with red locktite install the front set screw.  Done

Denny.

Denny,

This is exactly what I wound up doing with the addition of adding a tiny (the tip of a 5/32 drill bit) dimple to the barrel for each set screw, and it worked perfectly. Got it all assembled and dumped three mags through it this afternoon and it ran like a top.

Thanks.

Glad to help.

D.