I'm trying to answer the following question:
Compute the inclination and declination of the magnetic field that would be observed in Boulder, Colorado (40°N, 105°W) if the Earth’s field corresponded to a perfect geocentric dipole whose axis penetrates the Earth’s surface at 80°N, 72°W.
Source: question 4, exercise set 5.10, on p. 360 of Lowrie (2007).
I found the inclination using $\arctan(2\tan(40))$, but am unsure of how to get the declination.
Lowrie, W. (2007). Fundamentals of Geophysics, 2nd ed. Cambridge University Press.