The signal strength is 10 bars - so 8 bars should be 80%. I'm not sure why the digital channels aren't coming in so well, but it may have to do with your distance from the originating broadcast signal.
There isn't a way to get the exact percentage.
I am using an indoor antenna to watch tv. I've had the tv for about 3 months now, and the signal strength for digital channels has been horrible most of the time. When I check the signal strength, the most bars I have ever seen is 8 bars. But since it doesn't show the percentage, I don't know how bad or good the strength is. How many bars would 100% be? And is there any way of getting it to show the exact percentage?