Set up using chairgun

Always the muzzle my friend since the projectile isn't coming out the scope but the muzzle. Good luck and shoot safe

Except that holdovers and clicks are calculated from the scope position.

Some versions of Chairgun require the sight base distance, and some do not.

If the sight base is not factored in, for close shot (inside your zero) measurements, take it from the scope. For far shots (past your zero), projectile drop becomes more important, so take it from the muzzle.