To set the parrellax to a specific distance (reducing it from its preset), set the scope up so that it doesn't move, rested on a table or rest your gun on a table.

Point it at any object that sits at the distance you wish to use it at, ie 30 yards.

Move the front lens out slowly an eighth of a turn at a time, keep returning to the view through the scope and move your eye up, down, left and right and watch what the crossbars do.

As you approach the correct parrellax the crossbars will stop moving around at the point you see in the scope.

If you pass the point of no parrellax the movement will return again. If this happens.move the front lens in slowly to find the sweet spot.

This even worked for me on a pistol scope that was parrellaxed to some daft distance.