To get your rank and progress ingame, you may want to install the HonorSpy addon or execute the following macro:
/run _,r=GetPVPRankInfo(UnitPVPRank("player"),"player");r=r+GetPVPRankProgress();rs=string.format("%.4f",r);DEFAULT_CHAT_FRAME:AddMessage("PvP Rank: "..rs)
The estimation does not take into account honor stacking and assumes linear distribution of honor contribution inside a bracket (which would happen naturally without stacking). The calculator can still be used to estimate your progress if you are stacking. To do so, change your standing to the standing you are honor stacking on in your bracket (e.g. if there are 10 bracket 1 slots, and the top 9 players stack on standing 1's honor, the top 9 players may change their standing to standing 1 for calculation/estimation purposes). The projection is to be taken with a grain of salt, since usually the number of players and your standing are not constant. You are however free to manually estimate and enter your RP gains, the calculated result is then accurate.
In order for stackers to get the maximum amount of RP out of their bracket, the last standing of each bracket should stack with the next lower bracket. So, brackets span sufficient amounts of honor and breakpoints always fall on the honor of stackers. This effectively reduces the number of slots in bracket 1 by one.
This blue post explained SoM honor system changes:
A: There are RP (rank) caps per level. So if you got some HKs in SoM a few hours before the first honor update, you likely weren't on a high level. Below level 30, you are capped at 6500 RP total (rank 3.30). The calculator does not take your level into account and does not consider RP limits per level.
Some SoM servers launched a few hours before the first honor update. Players who took advantage of this and PvP'd in lvl10-19 WSG may progress like this (st1 capped to r3.3 first update and getting maximum RP the later weeks). Overall, still a minimum of six honor updates are needed to reach r14 from r0.