I/IV II/III IV/V I/M I/X
Tough getting all those digits on a scale or dial caliper though
On a more serious note ... from mathforum.org (quick search finds the geeks);
Did the Romans use fractions?
The Romans didn't have a standard way to write fractions using their numerals. Instead, they just wrote out the word for the fraction: for example, two-sevenths was "duae septimae" and three-eighths was "tres octavae." The Romans did not have a word for every imaginable fraction: how often do you need to say thirty-three seventieths? If necessary, they would probably have said something like, "thirty-three seventieth parts," or "triginta tres septuagensimae partes."
The Romans did most of their practical calculations with fractions by using the uncia. The uncia started out as 1/12 of the as, a unit of weight (the word uncia is related to our word "ounce"), but it soon came to mean 1/12 of anything. You can add up twelfths to make halves, thirds, or quarters, so the uncia was fairly versatile. When they wanted smaller fractions, the Romans usually cut the uncia into smaller parts. The system is very similar to measuring length in inches and fractions of the inch: you might not measure an object's length exactly, but you can still come very close.
There were Roman and medieval symbols for multiples of the uncia. The semis, which was six unciae, or one-half, was often represented by this symbol: . However, uncia symbols were never standardized, and not everybody used them. Some late medieval writers even substituted the modern fraction bar.