Your suggestion is good, but it isn't obvious to me how to assign weights to each position group. One way to would be to retrospectively rank each team's position groups from past seasons, compare those rankings to their team's conference records, and then find weighting parameters by fitting a regression. This is probably the best way to do it, but is pretty complicated.
Another less accurate (but perhaps easier) way would be to calculate the average NFL salary for each position and then use weighting parameters that are proportional to those salaries. This assumes that the NFL pays players based on their relative importance and that NFL and college positions perfectly scale. This isn't exactly true (RBs are considered more valuable to college teams than to NFL teams), but are still pretty good assumptions, IMO.
If we use the latter method, the salaries can be found
here for different NFL positions (warning: Sports Illustrated link).
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Here's the resulting spreadsheet. Instead of using the rankings, I computed a point value for each ranking, where higher numbers are better (11 - conference ranking = point value per position). A ranking of #1 equals a point value of 10. A ranking of #10 equals a point value of 1. Then, I multiplied this point value by the corresponding position. The weighted parameter for the DL was the average of DTs and DEs. The weighted parameter for the DB was the average of S and DBs.
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Here are the charts:
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