time makemkvcon --decrypt '--messages=.MakeMKV/Harry Potter - 1 - Sorcerer's Stone.log' --debug=-same --noscan --cache=2048 -r backup disc:0 "makemkv/Harry Potter - 1 - Sorcerer's Stone"
First, the --debug value is not overriding the one I have set in preferences.
Second, the --cache value actually used is 1024.
I have 16GB of RAM, so giving 2GB to makemkvcon is no sweat.

In an ideal world, though, I'd like to be able to put the messages/debug file(s) in the backup directory as well.