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I have found that when drilling holes that require a precise lay out , I would use a template. I recently fabricated aluminum backing plate for the interior of the transom. I used a borrowed template from my boat mechanic. If I were drilling mounting holes in the transom I would fabricate the pattern onto a piece of 3/4" plywood and attach a lip on the top to let it hang from the top of the transom. I would also install some square blocks on the bottom of the holes (screwed in from boat side to give a bearing edge onto a drill bit on the outside). Clamp the template in place. The plywood thickness prevents the drill from wandering and if you use an elongated drill bit, the bit rests on the block for keeping it at 90 degrees and you can "eye ball" square from the top by looking down on the block as you drill. I have used this method often for structural bolt layouts. I found that even with pilot holes it is extremely difficult to maintain a tight bolt pattern freehand.

Garris