um I don't think that is a good guideline. You can't keep a discus in a 10G tank...
I think a 55 is sort of a minimum for adults, and they need to be in groups of what, three or more? Something like that.
Anyways, I am not sure. I guess maybe if you kept 5 in a 55G then yeah that's 10G per discus, but that is the starting point I think.