The equation to determine gravity at a given latitude is supposedly:

http://en.wikipedia.org/wiki/Gravity_of_Earth#Latitude_model

However, i also checked the actual US Standard Atmosphere 1976 document (available here) and on the pdf page 19 they describe the value of g0 chosen for use in that equation.

Of course these are all estimates anyway, so unless you care about < 1% error it probably doesn't matter much.