AT+CSQ network percentage formula from rssi

Question

Does anyone knows formula for calculating GSM network coverage into percents (0 .. 100) from rssi? It should be safe for 8bit AVR microcontroller CPU, without hardcore math operations like log or division by something that's not 2^n (bitshift is preferred). Creating array with 32 possible percentage values is poor solution.

possible rssi values (0..31 is valid values):

0 -113 dBm or less
1 -111 dBm
2...30 -109... -53 dBm
31 -51 dBm or greater
99 not known or not detectable

Approximate values i want:

RSSI    %
0   0
1   3
2   6
3   10
4   13
5   16
6   19
7   23
8   26
9   29
10  32
11  36
12  39
13  42
14  45
15  48
16  52
17  55
18  58
19  61
20  65
21  68
22  71
23  74
24  78
25  81
26  84
27  87
28  90
29  94
30  97
31  100
99  ?

I'm out of ideas, so please advise me! Thanks for Your time!

Answer 1

(rssi * 827 + 127) >> 8

Multiply by 827, add 127 to simulate rounding to nearest, then drop the 8 low order bits, all in integer arithmetic.