Formula / Calculate how much to slow video down to match audio? scenario:

I have video matched with audio (song).

I slow the audio (song) down (eg from 120 bpm to 115bpm ie, I know exactly how much slower the new audio is than the original video)

How do I calculate how much to slow down my video by (in Final Cut I have the option of slowing down from 100% speed right down to 1%) so it matches the slow(er) audio?

thanks.
(115bpm / 120 bpm) * 100 = 95.83% (approx, to 2 decimal places)

Speed change percentages are always confusing.

Final cut expresses speed changes as:


  If you change the speed to 50 percent, your clip becomes twice as long
  [...] 
  If you change the speed to 200 percent, the clip becomes half as
  long


So if you go from 120 bpm to 115 (a slower tempo), you are looking for a longer clip, so your percentage should be lower than 100%.