Navionics water level setting
 

My understanding of this setting is when the lake is at full pool this setting should be zero, theoretically speaking. When the lake deviates from full pool, this setting should be changed + or - to coincide with actual depth conditions otherwise the reading on the app map would be incorrect.

So if you have an accurate depth finder on-board, setting the app to match your instrument would be the easiest way to get reasonable accuracy. Of course draft distance needs to be taken into consideration depending on where your transducer is.
Sound right?