The problem with a percentage cap is that it benefits those with the highest revenue. Which is probably one of the factors that caused us to withdraw from the ESL as they had proposed a 55% cap on wages and net spending so if we were getting less money than United or Liverpool, then we're at an immediate disadvantage.
Whereas in the NFL for example, there's a fixed cap. Think it's $182.5m this coming season. But the NFL do revenue differently as they negotiate not just TV deals but kit deals and major commercial deals. These get shared equally among the 32 teams. There's other revenue streams, including their own local commercial deals, tickets, the food & drink concessions, parking and other things that they can keep for themselves. So there's still a financial discrepancy between the top and lowest earning teams but that doesn't matter as they all have to abide by the same absolute cap.
As a result, Dallas Cowboys are the highest earners by a mile and, without the cap would win the Superbowl every year. But because of the cap, they've not won it for 25 years. So I've come round to the idea of a similar system here, with central kit and commercial deals negotiated by the PL and a fixed cap.
I'd also abolish the current method of transfer fees but have clubs pay off the contract of a player they want to sign. But that payment is included in the cap (although it can be spread out over the life of the contract).
Then we really will have financial fair play.