Christian is correct in pointing out EM isn't fixed in what it means for PX -- sure for 96dpi users 1EM == 16px, but on my machine which is set for 120dpi, 1em == 20px.
1) Avoid percentages on anything other than font-size, line-height, width or height. Using them on padding/margins can be a little... off.
2) if you change your font-size, restate your line-height! Even if you are setting it to the same value you CANNOT trust the inheritance... As a large font/120dpi user I see so many broken/unusable layouts caused by this simple omission where someone used EM to "help" accessibility for different font size users, but by failing to restate the line-height in fact flushed it.
In my code, the condensed property ends up about the same number of character as saying "font-size:85%; line-height:150%;" that I usually say "screw it"... and restate the whole condensed property... Well, compare:
font:normal 85%/150% arial,helvetica,sans-serif;
Oh noes, like ten extra characters... really the latter seems the most reliable and foolproof -- you'll see people take all sorts of shortcuts like omitting weight/style, omitting metric on the line-height -- and invariably it breaks SOMEWHERE. (usually on my machine!)... the full condensed property as above even if it's restating values is still the most bulletproof approach. Remember, they're called shortcuts for a reason...
3) due to rounding errors, 0.875 will give you 15 on some browsers, not 14px. It's why I use 85% instead... which also gives large font/120dpi users a more comfortable 17px instead of 18px. I suggest testing your math against at least the three main choices in Win7 -- 96dpi, 120dpi and 144dpi just to be sure of what you're getting, but always be aware that you CANNOT predict the user settings -- which is WHY you should be using %/em in the first place.
4) state your leading zero on decimals, some browsers may mistake that for a new class and bugger the whole property. (though I think the last mainstream browser to screw that up was either Opera 7 or Mozilla Suite 0.9 -- it's still bad practice)
5) even when using px, I usually state my bottom padding and/or margins in EM... not sure why. Usually I find 1EM looks best with a 140-160% line-height. (much larger than the default 110-130% you'll find across all the browsers)