How to locate Pitot tube holes in mast

How to locate Pitot tube holes in mast?

I'm installing the pitot probe and need to drill the four mounting holes in the pitot static mast.



The garmin probe already has the screw holes in it, the mast doesn't.

Any suggestions on how to locate the holes beyond very careful measurement?

Thanks for the help

Here's how I did it:

http://smilinpete.com/wp/?p=1645

Two

Use two lines instead of one. You will use the same technique when doing canopy skirts.