Yes that would would work also
It's good to know though about the Mac fonts being a tad wider.
Yes where you have something that fits by virtue of its content only then you can't rely on it being the same size cross browser as there are just too many variables to take into account not to mention basic rounding errors.
It's always safer to make sure the element either has width that fits when space is tight or add a few pixels leeway.
I often see small differences across platforms and indeed cross browsers so I always say just leave a little extra breathing space