I'm on my phone, so I'll find some links in a few hours... But if you look at some of my early build threads, I document my trials with this operation and there are references in them.
The basic plan I followed is this:
1 - get the most accurate drawing you can find for that part of the neck
2- plan your jig to place the hole at 3 degrees to hit a 3/16" hole where the edge of the hole lines up exactly with bottom of the truss channel. Use the estimated top of the fretboard or the back of the headstock for all your vertical references.
Roughly, the access hole edge should end up 1/8" down and in front of the nut when all is done .
3 - make a couple of representative necks/neck heads to practice on.
4 - make a 3/16" hole in the center of a 3/8 steel rod
5 - drill a 3/8" hole that leaves 1" to the channel
6 - use the hole in the rod to drill the 3/16" hole int the center of the 3/8 whole
Most of the process I do is in this thread - pages 7, 8 and 9:
Here's the best drawing for that area of the neck that I have found. Basically you want a 3/8" hole that ends in a 1" long 3/16" hole that ends in the bottom of the truss channel.
That is a deceptively challenging hole to make well enough to show off, but one that you really need to get right to get that easy access truss adjustment - which I think is pretty critical as I adjust mine about 4-5 times a year. I have gotten that hole too deep and it sat above the channel by as much as about 1/10" - that can be fixed by putting in a 1/4" shim along the bottom of the channel so that the rod will rest on it. Once you get more than 1/8" above the bottom of the channel you may run into problems with the skunk stripe staying in or even being thick enough in the middle without making the neck crazy thick. As far as getting right in the middle - I've gotten off center as much as 1/8" and it worked perfectly... Most necks I've done have gotten that hole pretty close, but there is a lot of forgiveness in that design as long as you plan and execute your jig for perfection and end up only 1/16" off somewhere.