The Levels# directive links hardware levels (bits per coordinate like 21 or 23) to zoom levels. For each object, the Data# directive defines the level at which the objects coordinates apply. My assumption is that the Zoom# directive maps the Data# to the Level#.
For example LevelX=21 ZoomX=Y DataY=(50.1,0.5) .
But, for safety, I keep X and Y the same so it's always, for example Zoom2=2
Have you thought of an iPad with a separate Bluetooth GPS device (I use a GNS2000) that you can position for best reception? Or an Android tablet with a separate GPS device? In UK the GNS2000 costs about 70 Pounds - so about US$110.