To evaluate the expression 5/6 - 3/4, we need to first convert the fractions to a common denominator. The least common multiple of 6 and 4 is 12, so we can rewrite the fractions as follows:
5/6 - 3/4 = (5/6)(2/2) - (3/4)(3/3)
= 10/12 - 9/12
= 1/12
Therefore, Devon used 1/12 of a cup more sugar than flour.