Personally I use a chalk pen. Put some liquid chalk (or any ink) on the tops of the posts, lower the shell into position and the holes are marked, I measure to check they are all even before drilling.
May not be the ‘correct’ way, but works every time for me. Also there is a tool for this that works on similar principle…
http://www.ebay.co.uk/itm/Body-Shell...item3a6823da4b