I looked this up a couple of months ago. There just aren't that many comparables when we talk about players who won the Art Ross Trophy during their entry-level contracts in the cap era. If McDavid doesn't win the Art Ross this year, he'll likely be a close second.
The only players to win the Art Ross Trophy while still on their ELCs are Crosby, Ovechkin, and Malkin. Of those, only Crosby had won prior to signing his second contract. Crosby and Malkin both signed their second contracts during the summer following their second seasons (with a full season remaining on their ELCs). Ovechkin signed his partway through his third season (when he was on-pace to win the Art Ross).
Since they all signed the season before the contracts came into effect, if you're looking at the percentage of the cap, you have to look at what the cap was when they signed, not what it was when the contract actually came into effect.
- Sidney Crosby signed his second contract on July 7, 2007, a month before his 20th birthday. This was after he won the Art Ross and Hart Trophies in his second season. He signed for a cap hit of $8.7 million per year when the salary cap was $50.3 million. That was 17.3% of the cap.
- Alex Ovechkin signed his second contract on January 10, 2008, halfway through his third pro season. He was 22. That was the season where he won his first Art Ross and Hart Trophies. He signed for a cap hit a little over $9.5 million when the salary cap was $50.3 million. That was 19.0% of the cap.
- Evgeni Malkin signed his second contract on July 2, 2008, less than a month shy of his 22nd birthday, after his second pro season. He finished second to Ovechkin for the Art Ross the previous season, and won the Art Ross and Conn Smythe Trophies the following year. He signed for a cap hit of $8.7 million when the cap was $56.7 million. That was 15.3% of the cap.
There's a good chance Malkin had to deal with a team cap imposed by Crosby's existing cap hit. If he had put up the same performance on a different team, he likely would have made even more than he did. Crosby also might have been able to squeeze out a little bit more if it wasn't for his weird "87" fetish.
So far, there's nothing to indicate McDavid has a similar weird obsession with his number. If the cap doesn't go up for next season, a contract with the same percentage of the cap as Crosby's will give McDavid a $12.63M cap hit.