If you really want to know how deep your dipsys are running per ammount of line, start trolling on a flat contour and keep letting out 10 feet of line at a time until you start rubbing bottom, then you know for sure. Then try running faster or slower and see how it affects the depth. They run deeper then the chart in the box states.
Calibrate your reels, Lance Valentine has a video on YouTube how to do it for those that are confused. Thats the only way you can replicate results once you find a pattern to the fish. Otherwise, sure you will be close but I like to be confident in where my lines are running. For my purposes the precision trolling app works, if anything my dipsys run a bit deeper then what they say they are, but again I know this because I figured out how much line to drag bottom.