There is something out there for NT7 that just plots the difference as a histogram on either side of zero depending on if + or - is larger. But I am assuming you use NT8.
Fat Tails has an indicator for NT8 , an improved version of Perry Kaufman's efficiency index. He says his improved efficiency index is superior for determining market direction, so that is something to be given serious consideration. However it only gives a single output oscillating between long, neutral and short. The numeric output is the same for short and long, but the line changes color.
I do not think, from my own experience, that you will make any money tracking the difference between the +and - DI's, as the higher the value the stronger the trend and they are not reciprocals. I have used the full DI system as a filter for several years, which means watching it on 1000 tick ES charts for the regular trading hours every day. They are a part of a system, not a stand alone component, so be careful out there!:)