Board Thread:Dragon City Questions and Answers/@comment-153.90.84.245-20130501153511/@comment-3271569-20130501162220

Rating = Total Earning Dragon A + Total Earning Dragon B

Example Legendary Lv 10 + Legendary Lv 8
 * Total
 * Total
 * Total

Slots: Default= 1:1:1
 * 1: 50 + 5*9
 * 2: 50 + 20*9
 * 3: 100*10

So 760
 * 1: +10 = 11
 * 2: +10 = 11
 * 3: +7 = 8

Chance
 * 1 = (11/(11+11+8))*100 ~ %
 * 2 = (11/(11+11+8))*100 ~ %
 * 3 = (8/(11+11+8))*100 ~ %

Note that the system is still random. This is only the ~chance.