If you do not have the adjustment screws, and a 2007 DS likely does not.
When you have emptied the grey water tank ; while holding the Grey water button, press the E button until the level lights flash.
When the Grey water tank is full, do the same except use the F button.
The other tanks calibration are set the same way.
Here is a link to the manufacturer of your panel.
(Smart 102 - 3 and 4 Tank with Push Button Calibration)
If your panel membrane cracks you can get a replacement there for $10.00
https://tankedge.com/techsupport.html
Also have circuit boards and tank sensors.
Good Luck!