Or use a glyph of some kind. I think in the car I'd be perfectly OK with 1-5 bars, like on a phone. That would always be the same width and if it appears first you won't have to worry about populating two columns. It looks like the GuiFX font has some that would work well.
Personally I like the percentage. Perhaps clicking on the "glyph"(never heard that term before) would toggle to the percentage.
I've got a bigger problem anyway... I cant get a 'clean' list of access points. I am using WMI and it returns garbage SSIDs. I think there is a bug in WMI because I am pretty confident my code is okay. Even the WMI tools (object manager) report back garbage.