Does that mean 1/3 of his shots can be expected to go in?
I see your point and checked. Of the top-15 goal scorers this season, none ever reached 20% (best was Stamkos with 19.8% two years ago).
So with 29.7% Marchand is the best in the league this season.
If he would keep that pace, he would break records for years if not decades.
With a very good 15%, Marchand would have 6 goals not 11 goals.
So the question is: will he be the best %scorer ever?
I guess expecting a pace of 20% (best since previous lockout) is the best that could happen (meaning the his scoring% is the best of every star in the league for the last decade).
20% would be 8 goals, 15% is about 5.5 goals.
So either Marchand shoots 50% more or he gets down to 15-18% (5-7 goals instead of 11) pace.