I did mention in one of my previous answers on big Roman numbers (https://www.reddit.com/r/AskHistorians/comments/lj6jfz/why_did_the_roman_numeral_system_only_go_up_to_m/gnav2nf/?context=3) that :

Roman also had some numbers lower than 1, as they used S to denote 1/2 and a dot to denote 1/12. There are a lot of other subtleties, but putting all of them here would just be repeating the wikipedia page (https://en.wikipedia.org/wiki/Roman_numerals).

So ".." would be "2/12 = 1/6", "S..." would be "9/12 = 3/4", etc.

More detailed answers from other users are welcome, as fractions were not the subject of the linked answer.