I installed my sharkfin just now. I am usually pretty obsessive about following instructions but after assessing the situation I made an executive decision to change up the process a bit and I don't think I lost anything in terms of alignment or adhesion.
Instead of putting tape all over my paint to mark the position, I used a single 3"x3" Post-It note at the rear of the fin to mark my position after centering it up to my satisfaction. The Post-It wasn't quite the width of the fin, but it was close enough that it was very easy to eyeball it and center it properly by gauging the overhang on both ends.
With this method, it makes more sense to apply the rear of the fin first (vs the front as per the instructions). This is what I did. Theoretically I suppose it is possible that this could produce a weaker grip on the front of the fin but I am willing to live with that. I addressed it by making sure I held firm pressure on the front and all around the fin to activate the adhesive. I think I got a strong fit.
If it peels off at highway speeds you guys will be the first to read my red-faced report.