Exact mechanical location of pin1 on J8 of raspberry pi 3
This was right on google, but I'll post it anyway:

https://www.raspberrypi.org/documentation/hardware/raspberrypi/mechanical/README.md

You have PDF's and DXF's the DXF's should prove most useful if you have a reader, because you could use a cad program to draw dimensions on the DXF.



Keep in mind its just a 0.1" header, they are pretty standard, the drawing only gives the outside dimensions but the inside dimensions can be calculated if you use some intuition and logic on the centerlines and the outside of the rectangle. I don't know if the mechanical drawing given is the silkscreen or part outline, but if you go off of the centerline it should be fine. Keep in mind there are a few mills of space for through hole parts so your tolerance on that dimension will be roughly 5mils.