spacetimeengine

Yes, the formula is the Einstein's Field Equation. Alas, I don't think it's generally considered as nice and simple. But it's the tool for that: you would basically set an energy density distribution and the software would calculate the metric of spacetime (and that would include time distortion) I guess you can find repos on Github (first Google result for me was this: https://github.com/spacetimeengineer/spacetimeengine) and try your hand. But the theoretical aspect is intimidating, for sure. As for getting up to speed in the theory, I cannot but recommend Leonard Susskind lectures on GR that are available on YouTube. Susskind may be best known as one of the founding fathers of String Theory but the man is an exceptionally great teacher on this subject.