I do wish people would stop recommending the setting should be fixed on a certain value, that's not how it's supposed to be used.
It's meant as a 1 off thing for power users.
Once you've fixed it on a custom value, it'll use that for everything you ever burn - regardless of correctness. If you forget to change it from this fixed value when you burn a different DL project, tough, you just burnt a potential coaster.
If you leave it on Auto, it'll use the LB value as taken from the .DVD or .MDS file, and if no such thing exists, it'll try and find the best one by scanning the content of the image (or files if you're using Build mode).
So basically, leave it on auto and trust the program that made the image and/or ImgBurn to do what's right.