The rear set screw is normally directly opposite the hole. You can take the gas block off and remove the gas tube. Remove the set screw and slide the block back on upside down. Center the set screw hole over the gas hole in the barrel and measure any gap needed. When you reinstall the gas block a feeler gauge can be used to get the same gap. If you lightly twist the block as you tighten the set screw you can feel the set screw as it centers into the dimple.
If a gas block is touching a hand guard you usually get some random flyers in odd directions. Stringing might be more related to the fit of the upper, barrel nut. and barrel.