There are two 8/3 settings possible on the rear sight.

There's 8/3 as in 800 meters and 8/3 as in 300 meters.  Sight bottoms out (all the way down) at the lower setting.  Sounds like you had it set to shoot at 800 meters (all the way up).  

Top 8/3 - 5 clicks is still about 725 meters and gonna be very, very high at 100!  Like about 2 feet.

Mechanically zero the rifle again:  Rear sight at 8/3 (all the way down, not up).  Base of front sight flush with top of front sight base.  Zero the rifle again.

Do NOT zero at 100 meters, your rifle will always shoot low.

Once you're sure your rifle shoots properly search for Improved Battlesight Zero (IBSZ) and try that.  Very flat trajectory for all combat ranges.  Essentially a 50 yard initial cross over with the rear sight reset so the range wheel is meaningful.

-- Chuck