Correct, approximately, and... not like that
I think it goes something like this...
Start with what you would get from your position when everyone has equal DR: (3 - 12) * 80 = -9 * 80 = -720
Then subtract that from what you actually scored, and multiply by 500: (-414 - -720) * 500 = 306 * 500 = 153000
Then you can get an approximate average DR difference compared to you, by dividing by the number of others: 153000 / 15 = 10200
Then an approximate average DR for the whole race, by adding your DR plus your DR divided by the number of competitors: DR + 10200 + DR / 16 = ...
It's very approximate because of all the rounding to integers that's done in the scoring, and because it rounds down that means the error is always in the same direction. In the race I tried this method against the average DR estimates were about 1700 too high (13 competitors, so about 130 per), but were within a few hundred of each other.