I haven't seen this posted before, but I took a shot at how Chronopia calculates the DEF score. On page 50 of second edition it states that DEF is calculated from CC and AR value. I took a look at blackbloods mostly because they came first in the book and noticed this relationship:
AR value Calculate using
25-26 DEF=AR-CC-11-Shield Modifier
23-24 DEF=AR-CC-10-Shield Modifier
21-22 DEF=AR-CC-9-Shield Modifier
19-20 DEF=AR-CC-8-Shield Modifier
17-18 DEF=AR-CC-7-Shield Modifier
15-16 DEF=AR-CC-6-Shield Modifier
13-14 DEF=AR-CC-5-Shield Modifier
12 DEF=AR-CC-4-Shield Modifier
For mounted troops, use the rider's CC. This breaks down in 2 cases.
First, for the ogre myrmadon, double mounted kutara and goblin chariot, the def was worse by 1 than what you would expect. The kutara lancer and goblin ripper beast riders, however calculated using the above formula. This means that very large mounted units and chariots have a worse DEF than you would expect.
Second, very small units, such as warhawks and darkwings, have a DEF score that is better than what you would expect.
Notice also that this fits in well with the devout spell corrosion, in which a models def improves by 1 for every two AR it loses.