To quote Magus:

"Trajectory DI effect in degrees is the perpendicular distance, squared, then multiplied by 18. DIing Jigglypuff's u-throw [which ejects at 90°] at 0 damage with inputs of 0.7000,0.0000 or 0.7000,0.7000 both result in 0.62238, 4.01108 launch speed on 1st frame of KB, which is an angle change of 8.82 (0.7 * 0.7 * 18) from 90° -> 81.18°."

There are as many "notches" of DI as there are inputs on the control stick. The control stick inputs are all ordered pairs (x,y) such that x,y ∈ ℤ; -80 ≤ x,y ≤ 80; and x^2+y^2 ≤ 6400.

Schmooblidon's

knockback calculator also has a DI calculator in it. You can use that to figure out what input would maximally influence your trajectory in a certain direction. The "strength" reading right below the control stick input circle tells you how much of the maximum possible 18° that DI input will cause your trajectory to deviate from the base trajectory, e.g. a strength of 50% would yield a 9° difference from the base trajectory.