25-30 sounds about right.
pellet weight, pellet BC, scope height and of course the velosity/power makes the diference.
check out chair gun, have a play with it so you can see why.
(the mildots come into it when you set chairgun up).
finaly...you could go with the 30 yard zero, then set up target every 5 yards to see where the pellets are hitting, but quite often a diferent zero sets the tragectory arc a little tighter on your cross hair.. depends what your into really.. or how heavey the peelet is.. say a heavey pellet at 30 yards, a lighter mid weight at about 27, a very light