I made this ages ago, it's super easy to use. Basically, here is how it works:
Google Docs of the Spreadsheet
There are 3 red fields which you can edit, do not touch the rest (they're protected anyway).
The amount of players is how many players are in "Standing" each week. On Alliance it's currently between 5000 and 6000.
Once you have entered that, it show you the amount of players getting RP for every break point. For example, if you NEED 12000 RP next week and the "Amoutn of Players" is 6000, you need at least standing 18 to get that many next honor calculation.
The RP needed for rank just shows you how much RP you need to reach any rank. They're flat values and only serve for orientation. By entering your current rank and the % you are into that rank, it will show you what RP you will have next week, depending on your minimum standing you know/think you can reach (from experience). Remember, you lose 20% of your "Current RP" every week, before you gain extra from honor calculation.
So let's assume a scenario here to make this more obvious:
You are currently rank 4 and 50% into rank 5. You wonder if you can hit rank 6 next week. There are 5500 players in standing on your faction. This means, your current RP is 12500 (putting you right inbetween rank 4 and 5). Therefore, if you can reach 15000 RP by next week, you will hit rank 5. After entering your data in the spreadsheet, you see that you will need at least standing 1254 to achieve your goal. You decide this seems easy enough and aim straight for rank 6. Turns out to gain now you need at least standing 110 to reach rank 6.
Basically - this helps you to calculate exactly how much effort you need to/want to put in every week to reach your desired standing in time and whether you can actually reach it at or if you will just drop every week, because you cannot even keep up with the 20% of your current RP.
Google Docs of the Spreadsheet