Great circle formulae, True cource and actual heading

Hi,

I am trying to write a formula to compute flight headings.
If i use 2 lat/lon i can get a "true course" but due to magnetic variations
the true course is not really all that useful, (knowing that my true course
is 118deg will not tell me that i should fly 93 deg).

What i mean is if i go from A to B i can calculate the True course, ('tc')
but my actual heading varies a lot depending on my position along the
course.

Is there a reliable formula to calculate my actual heading at a certain
point along the course?

Many thanks in advance.

Sims