multiavchd : AVCHD compilation won't fit BD-R

[rs008f]

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.

[rs008f]

I also notice each time I click on "fit all", it deletes the transcode settings I have made earlier so I have to redo it again.
Edit: I found a solution. I use notepad to automatically replace the settings so I don't have to manually click.

[rs008f]

Setting the custom size to 22000MB fixes the problem when there are more than 50 files.