multiavchd : AVCHD compilation won't fit BD-R

I set the compilation to fit a BD-R but the end result is 23.5 GB which won't fit a BD-R. Is the calculation of multiavchd inaccurate sometimes or is it due to my background audio file size?
Should I use custom size of 23000MB instead for future compilation? I notice if i click "fit all" a second time, a different compilation estimated size will be calculated.