Assuming the mobile device uses the 16px 96dpi default, 0.7em would be 11px -- as a rule I wouldn't go below 12px on ANY device unless it's an ancient 75 dpi device (and even then the only time I'd use 12 is as all-caps) so that's automatic /FAIL/... keeping in mind that some mobile devices start out at 14px, that's an even bigger /FAIL/.
Generally I use 85% everywhere, that gives 12px off the 14px 'smaller', 14px at 96dpi, and 17px at 120dpi (thanks to rounding which is why I use 85% instead of the 'perfect' 82.5%) for sans-serif fonts... and 100% for serif fonts (though it really hinges on the font face since they aren't all sized the same).
Though kvetching over size from 85% to 100% is almost pointless when you can't guarantee the delivery of the exact font you design for, the same font renderer, the same font kerning (yes freetype, I'm looking at you!)... I mean with widely varying font size results like this:
We're really spinning our wheels on the subject as simply changing the face changes the rules! It also shows why I'll say 85% on sans-serif and 100% on serif... except of course Verdana which is uselessly large compared to other faces; I think that's the real reason some people find verdana more 'legible' at the same 'size' -- as it's actually larger when you declare the same size. Segou UI is just as 'legible' if you bump the size 25% so they're near identical.