Champion Mastery: Grading Distribution

I've recently become a little more curious as to the specific ranges that determine each grade. From https://support.riotgames.com/hc/en-us/articles/204211284-Champion-Mastery-Guide we get:

We take a series of performance metrics and compare how you did to all other players in your region in that specific champion/position combination.

This is percentile-driven; earning a high score means you performed in the top x% of players in that champion/position combination.

I just searched for specifically what percentage of players score S+, and got top 1%, but can't find information on the values for anything else (or, for that matter, find those discussions again).

I'm looking for something like:

D-: <1% D: 1% - x% ... S: y% - 99% S+: >99%

(Is the distribution symmetric? I think it's symmetric.)

Riot please.