25/21

25/21 |0 -1 2 -1>

301.8465 cents

sound sample

25/21, the quasi-tempered minor third, is the interval between 6/5 and 10/7, and also 7/5 and 5/3. It is 126/125 flat of 6/5, and tempering 126/125 out makes the 1-6/5-10/7 triad into a diminished triad where two minor thirds make up a tritone. It is extremely close to 3 steps of 12edo (300 cents).