So i'm shooting a music video in about 12 hours and I really want to use the slow mo effect. I want to shoot the slow mo parts in 60fps and edit everything else in 24fps. So my question is, how much do I speed up the backing track for the artist to play/sing to in order for it to sync up correctly? I read 240% somewhere but that's insanely fast and not possible to even comprehend! Please help!
It doesn't work like that. The camera shoots silent and the audio is either recorded on-set with an external device, or usually it's just a dub of the studio track with the band miming to playback.
60/24 =2.5, so you'd need to speed up the on set playback to 250% in order to have the slow motion lip sync at 24 fps. (This is theoretical. I've never actually done it, so you should test it first.)