You don't need accuracy to better than a minute of time? It's hard to do much better than that anyway, since refraction can't be exactly predicted.
I'm not home now so I can't look it up myself, but Jean Meeus gave formulas in his book Astronomical Algorithms, which should suffice.
Calculating the sun's declination is easy enough as I recall... the equation of time is tougher, but still doable if you don't need it to the second. Then it's just a couple of straightforward spherical-trig formulas.