Depends on how much you want to shorten it. A few inches should be possible by following instructions for set-up found on this forum. Look for a "Holy focus" thread by Guy Kuo. In the special guest forum, Chris Stephens also gave somewhat similar instructions.
Basically, by making sure you install the pj correctly (level, parallel to screen, needing as little correction as possible, etc), and making sure you are using the better part of the phosphore area of the tube, it very often happens the distance you need to fill a given screen is shorter than the documented distance. The manufacturer distance often plays it safe.
Now there is no way to calculate that beforehand. The method is put the pj on some moving support, to maximize the phosphore usage, to turn the thing on and then to move the pj until the screen is fully illuminated.
I just described here in a few words a procedure the above guys need a page or two to explain, so I'd strongly recommend you check the posts of the experts before attempting it.
If you give the Barco model you have, other members may have better ideas. Maybe the lens can be changed?
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