The sensor only turns the screen off if it detects an object not farther than 5 cm. I did find that turning the low-power screen off saves on battery... 1 or 2 mAh or so. :P You could use bmedatalogger to compile a list of battery parameters across a period of time and then turn the low-power screen on and off, use a 2G or 3G connection, dim the screen etc.