It's ALMOST linear atleast:
Rank difference | % increase in RR | % change from previous rank bonus
40 0,00% N/A
39 1,26% 1,26%
38 4,15% 2,89%
37 7,04% 2,89%
36 9,93% 2,89%
35 12,82% 2,89%
34 15,71% 2,89%
33 18,60% 2,89%
32 21,49% 2,89%
31 24,38% 2,89%
30 27,27% 2,89%
29 30,16% 2,89%
28 33,05% 2,89 ...