There are 2 major breakpoints. 70% and 120%.

Below 70% you have a "fast roll." Above 70% you have a "fat roll." And above 120% you cannot roll or backstep all. These breakpoints affect the speed of the rolling animation only.

Roll distance (as well as stamina generation) is calculated in a linear way, there are no hard breakpoints. That means that every percentage point has the same effect as any other. Going from 69% to 68% will give you a very slight improvement in roll distance and stamina regen - going from 12% to 11% will give you very nearly the same degree of improvement.

Therefore, moreso than in the other DS games, your encumbrance is really up to your preference. If you go super light, you will have the exact same rolling animation and iframes as if your weight was 70%, but you have longer distance on your rolls and better stamina regen. The balance is up to you. It's worth keeping in mind that some enemies/bosses can actually get more difficult if you have a longer roll since it will roll you out of range to attack.