In this paragraph, a few best practice rules on drafting fractions are discussed.
15) Avoid using fractions.
When fractions are used as adverbs or adjectives, insert hyphens. Do not insert hyphens if the fraction is a noun. In a closely linked phrase avoid combining fractions and words.
For example:
a two-thirds increase
increased by two thirds
two-thirds completed, but not: ⅔ completed
Never contrast or compare a fraction with a decimal (i.e. avoid the applicable interest rate shall be 6¼ percent instead of 5.1 percent).
16) When used, fractions should always be spelled out in words, even when the figures are higher than ten, unless they relate to round numbers.
For example:
The JVC shall budget one tenth to developing its distribution network, a twentieth to R&D and no more than a thirtieth to raw materials.
Fractions are subject to the general drafting principle of ‘be accurate’; in many cases, fractions are more precise than decimals (e.g. 6.33 neglects the implied infinity of the fraction, which is embraced by 6⅓). Alternatively use fractions for rough figures, e.g. the JVC shall make best efforts to decrease production costs by 3½ percent a year, and decimals for more exact figures, e.g. the retail price index is rising at an annual rate of 10.6 percent.
Note that 2.5 (as in 2.5 percent) is not the same as 2.50 or 2½! Each decimal place, even if zero, adds to the accuracy. The non-decimal fraction is more approximate.