if youre showing -91dBm and getting 1 or no bars then thats pretty accurate......... -80 to -90 is the general operating range for cell signal....... you can probably still be getting enough signal to be useful up to about -95 but beyond that its roaming or nothing typically.........
you should be seeing 3-4 bars as you hover around or below -80 and obviously the lower it gets the better signal youll have
as far as the 50% bug goes...... its a reporting error only....... Ive done a test on my phone using the following method:
for 3 days: normal use, no airplane mode, 50% without signal bug, put on and took off charger at specific times, noted amount of battery left when placing on charger......
for 3 days: normal use, using airplane mode trick, 0% without signal, put on and took off charger at same specific times, noted amount of battery left when placing on charger
the 50% bug had absolutely zero effect on my battery....... its strictly a reporting error in the coding
and though youre late to the thread its still relative...... especially since there have been 2 other threads just started in the past couple of days on the same topic