I think Sid's second contract is a perfect comparable.
Crosby signed it the summer after his second season in which he won the Art Ross, Lester Pearson (now known as the Ted Lindsey) & Hart. He signed a five year deal for 17.3% of the cap hit at the time
according to CapFriendly.
McDavid is in the summer after his second season in which he won the Art Ross, Ted Lindsey & Hart.
If he signs a five year deal for 17.3% of a $75-million cap, it would be $12.975 for five years (for a total of $64.875).
I wouldn't be surprised if he lands about there. He would be taking a bit of a discount, but not the ridiculous amount suggested by some Oiler fans. They are probably starting off asking for $14 million or even more.