Looks a bit too low.
The Salary Floor for my league is about 10% of my player salaries now.
Well I just promoted, and the PSF for me is 65% of my salaries.
I'm sure some teams can promote with less salaries than I did, so I don't think it's as cut and dry as you say.
Yes it's low, but you can't force promoting teams to buy players, either.
"Air is beautiful, yet you cannot see it. It's soft, yet you cannot touch it. Air is a little like my brain." - Jean-Claude Van Damme