I found a convenient rank calculator for SoM that is up to date and even has settings for the time when the ranking was screwed up in SoM.
Enjoy.
https://www.vanillawar.com/tools/honor/How do you know pool size?
honor spy addon will log players pvp stats and give you an idea. OR go with the OG vanilla method and create an alt and attempt to gain 15 honor from 15 hks so you are at the bottom of the pool and when reset comes around it will say standing xxxxx and that will be the aprox pool size
F = (1-d)^n(r-(1/d)(w)) + (1/d)(w)