[pct-l] Halfmile's mileposts and Google Maps

[James F. Miller]

The North350Blades, the PCTA trailcrew for the north 350 miles of the trail use Google Map coordinates to report our work. 
Some of us also use Halfmile's information for length of work done and also to give us PCT trail distances to our access points (roads that cross the PCT).
My question is: 
If I have the Google coordinates is there an easy way to calculate the Halfmile location. 
Presently I just get both readings in the field.

Bigfoot Jim