Are you talking about when the season ends and salaries get renegotiated during the off-season? Because the DMI fluctuates as the season progresses and therefore your theory would not hold true at all times.
it fluctuates a lot, during training or different form ;) With proficient form it is nearly constant, the value isn't totally correct but a good estimation.
Another thing you could use the DMI was to find out the sub of the game shape from your players, who don't get training.