Not saying this is the best way to go about it, but if you want to be statistical about it and more realistic than we have now, then you would simply gather data from the past 10-15 years+ and track each players school, measurables, draft position, and career stats. See the correlation between them and create probability based progression and regression that is behind the scenes.
The data is all out there, gathering it won't be a huge issue, neither would plotting it. It is the number of players that would be the biggest time consumer, but getting the data and plotting it to find the probabilities needed would not be very complex and it would be more accurate than anything we've seen in the franchise yet.