The main problem is that these calculators are either not updated or use the wrong information. The cap is based on your best personal rating of the week. Players who do not understand this and submit their team rating or best personal rating at the end of the week instead of their best at any point in the week can greatly affect data.
The only sure way to calculate this is to take your own data and not trust anyone else's. Make sure you track your personal rating after each game so that you know what's your best rating of the week. 3-5 weeks should give you a decent estimate if you stay around the same ratings. You'd need a lot more to cover from 1500 to 2500.
Here's the best I could come up with and it's probably not very accurate over 2100, but +/- 25 points if you are 1700-2100. Added RBG with *1.22. Not sure how accurate, but fine for now.
Rating -> Arena -> RBG
1700 -> 1675 -> 2044
1750 -> 1750 -> 2135
1800 -> 1860 -> 2269
1850 -> 1950 -> 2379
1900 -> 2025 -> 2471
1950 -> 2105 -> 2568
2000 -> 2180 -> 2660
2050 -> 2250 -> 2745
2100 -> 2315 -> 2824
2150 -> 2375 -> 2898
2200 -> 2430 -> 2965
2250 -> 2480 -> 3026
2300 -> 2525 -> 3081
2350 -> 2565 -> 3129
2400 -> 2595 -> 3166