I think basing a high school recruits ranking on potential in the NFL draft is patently absurd. We are about 3 months from the draft and even with the NCAA football season over, the draft board is going to be as tumultuous as ever...doing so 3+ years out is impossible. Even if you follow general trends, interior OL routinely get drafted in the 1st round anyways, so that argument fails as well...I am not going to bother with the research, but I have seen plenty of interior OL come off the board in the 1st round over the last 10 years.
Nonetheless, I think rankings should simply reflect how good of a player you think that prospect will be at the college level. Martell is a great example. Do I think he is a future high-round pick? In all likelihood, probably not. Do I think he could absolutely light it up at the college level? Absolutely and I think he will. NFL success is great and all, but it is not necessarily congruent with success at the college level. Great example is Troy Smith...not that he was a 5 star recruit (4 star by most), but he was one of the greatest college players I ever watched. That in no way correlated to his NFL draft pick, but he had about as big of an impact as you can have at the college level.
In sum, just project the kid to the next level in college...that is difficult enough as it is.