Not sure whether I understand the question, but whenever you deal with rational numbers and do not want to lose precision in calculations you want to use Gmp.mpq, rather than floats, doubles or Gmp.mpf:s.
Or, if you were referring to my particular current application, I am toying with an SVG optimiser/pretty-printer. (It's in good style not to degrade the quality of the input file.)
/ Johan Sundström (Achtung Liebe!)
Previous text:
2004-02-07 08:07: Subject: sscanf and Gmp.mpq
When do you need it?
/ Mirar