Yup, Robin2y , that's how it works.
Ideally, even with the 18.5" barrel , your 100 yard zero should be 8 clicks up from bottom on the rear sight.
It can be tricky achieving this with the inconsistancies of standard chinese front sights.
I have a client's rifle, a krieger barreled 18.5"
With rear sight bottomed out, it shoots approx 1" inch high at 100. The rear sight is perfectly functional, we're keeping it on the rifle. The front sight and flash hider are being replaced with a SEI DC Vortex and SEI gas lock front sight set, purpose made by SEI for 18" barrels, the client is supplying these parts.
Here is an opportunity for folks like NEA and Dlask to manufacture some metric , castlenut retained muzzle devices with no front sight and a gas lock type front sight, metric and set to the appropriate height for 18.5" barrels.