In a perfect world this is a great idea.A salary cap as a percentage of a clubs revenue would be a good idea. It would mean clubs only being able to offer wages that they should be able to afford to pay after other expenses have been paid. Clubs that are making good commercial decisions get the reward to pay bigger wages.
In the current world this is more or less what the FFP rules mean to do, but the actual situation is that a few teams inflate their revenue with bogus sponsors so this makes the field even less even - Chelsea, City, Newcastle currently can never go bust using the same rules as their sponsors can pump in as much as it's needed to balance the books. Sky money also make this incredibly lopsided as they give more money to the sky 6, etc.
We were mostly all against it but the super league sounds great... for the teams outside the super league. Then those rules can be applied while Real play PSG for the 63883th time this year.