Please confirm that I am going the right direction before i waste a bunch of time. I mounted a new scope on my  PSL and tried to get it sighted in. I couldnt get on paper to save my life so I decided to move closre and ended up and 25 yards for a starting point. I slowly worked my way back to 100 yards and began to work for zero. Well to make a long story short I ran out of up travel and it is still shooting about 3" low. I  put some shims under the rear of the rail.  I probably wont get out to shoot it for a while now. Just want to make sure I am going the right direction

Any chance you have a 20moa rail mounted backwards?

Lots of scopes do not have a lot of internal adjustment, some do.   They make scope bases in flat and Plus 20 MOA etc a Plus 20 MOA adds 20 inches above at 100 yds.  Like the guy above was wondering - is there any chance you have a Plus 20 MOA base mounted backwards where it added 20 inches below.   If not then using a correctly mounted Plus 20 MOA base would add 20 inches at 100 yds   (above).  It's better to have a scope not be at the extreme end of its adjustment range.  Shimming the base by adding a piece of aluminum pop can can give you the few inches you need that would be under the back part of the base but not as good of long term plan as a plus 20 MOA base.

Its a cheap POS scope from Walmart just to try out an larger than factory optic before I spent any money on something descent. I donty think this mount was available in a 20moa built in