Firstly with the bonnet catches you need to be sure what locks you are using before you drill the final holes, flush type or T handle as the holes are different.
The T handle has a round hole with 2 side locator's.
The flush type has a round hole with squared of corners.
The first measurement is taken 3" down from the center of the bonnet. I knew exactly where mine was as I placed my first dome screw for the bonnet scoop dead center (see older post on mounting the bonnet scoop )
This line extends 14" out from the center of top lip of the scoop.
In the above pics I took 2 measurements one in a straight line from the center of the scoop and one at an angle to meet the second measurement 1.5 " in from the edge of the bonnet at that point.
There was only a couple of mm of difference so not that important what way you do it.
Once I got the cross where the 2 measurements met I then transferred the measurement underneath just to check.
To make sure these measurements are correct next you need to measure the bulkhead lip (20mm on mine) plus the escushion 1mm, then subtract the bonnet gap when closed (7mm on mine) This gives a measurement of 14mm so allow 15 :)
In the above pic you can see a thick red line that has been drawn 15mm in from the edge of the bonnet, this is the bulkhead line plus the escushion plate measurement.
You can see that the widest part of the locking mechanism is at this point and the barrel part is well clear of the bulkhead and wont foul.
The pilot hole is then drilled,
The same is done with the other side.
I cannot go any further with mine at the moment as I have T handles and want to install flush locks. Next job to do is drill out with a hole saw the hole for the barrel, drilling through one skin from the top and one from the bottom.
Once the lock is fitted. the bonnet will then be packed level with some card then the locator's slots can be cut out in the bulkhead. Don't forget to order the escutcheons for the locator's slot like me :)