Hi N2 DPMS has a chart on their website on how//where to measure what hand guard height you need...I even think there is one here...one of the guys will chime in ....the best/lightest would be in my book the apex hand guard....light and well built...i think their site has the chart too.... if you have a rifle length gas port,get the long rifle length hand guard to cover the gas block....when looking to cover the gas block measure to see where it is located then find your hand guard length at what ever hand guard you want taking into account the height for your DPMS upper that you have ,because DPMS has 2 heights
and yes you measure towards the muzzle from the front of the upper where it meets the nut
:) Wash