Calculating distance from center of mass of two stars in a binary system
It actually doesn't really matter: the angular separation is sufficiently small that you can use the small angle approximations $\sin \theta \approx \theta$ and $\tan \theta \approx \theta$ (where $\theta$ is in radians). In both cases the next term in the Taylor series is $\mathcal{O}(\theta^3)$ and the effect on the answer of dropping this and all subsequent terms of the Taylor series is in most realistic cases going to be utterly dwarfed by the uncertainty in the distance.