Formula / Calculate how much to slow video down to match audio?
(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%.