If you are going to be slowing down the video by 2x, you speed up the click track/music by 2x.
So let's say the video is shot at 60 fps, then it's slowed down to 30 fps. Since the final framerate is 30fps, that's also what the music will be synced to at normal speed. Let's say the music is 120 bpm normally. It's rate has a 1:1 relationship with the music video's final framerate, in this case a clean 30 fps. So if you are shooting at 60 fps: 30/60 = 0.5
That's the ratio, so to get the bpm you want to use when shooting, just divide your starting bpm (120) by the ratio. 120/ 0.5 = 240
If your final framerate is 29.97, but you are shooting at 60 fps: 29.97 / 60 = .4995
So with a 120 bpm track: 120 / .4995 = 240.24024
It's a linear relationship. You just need to know A) The original framerate B) The final framerate C) The original bpm
EDIT: From the comments
[new bpm] / [old bpm] = [original framerate] / [final framerate]
Solve for [new bpm]
a / b = c / d
a = bc / d