While perfection isn’t absolutely necessary with a shot-ski it’s nice to come as close as you can to uniformity. First, pick the number of shots you’d like to have on your ski. Four is a nice round number and a good starting point.
If you must have more than four shots on your ski, by all means go for it, but cramping the ski will probably only lead to awkward shot taking and more booze on people’s faces than in their mouths.
A good rule of thumb for spacing is approximately 18 to 20 inches apart. Good spacing will allow for a more comfortable forward-facing approach to the ski, which is already a difficult thing to deal with. Make marks that are centered in the ski widthwise and get ready to drill.