ratio with the genitive case means “a reason for…” What usage of the genitive is this. It can’t be possessive or subjective, so the only solution I could come up with is that it is some sort of objective genitive, but this doesn’t make sense because ratio is not a noun of feeling or agency.
“A system of thought”
“A plan of action”
You’re right, it is an objective genitive. I’ve been staring at Caesar for the past week writing my honors thesis, and he uses it all the time.
The main idea is that the ratio is taking something in the genitive as an object, which occurs with nouns that imply a feeling, action, or agency (from Allen and Greenough).
Think of it as if you could use a noun as a verb (like amor for amo) where the verb would take a direct object in the accusative, the noun takes an object in the genitive. Thus: ‘Amo patriam’ = ‘I love the country’, and ‘Amor patriae’ = ‘Love for the country’
Ratio here is implying an action/feeling here, as it can be taken as ‘consideration of’ or ‘theory’ or ‘judgement’. All of those ideas can take an object, and thus the objective genitive. I believe it derives from reor which is ‘to think, deem, suppose’ (not sure on that though).
For more info, you can look at Woodcock’s “New Latin Syntax” section 72, and 74-76. Though, he does not look at ratio specifically.
Did this help you any? If not, I’ll try harder! 
That makes sense. It is sometimes difficult to determine exactly what nouns can be considered nouns of action or agency.
Yes indeed, you speak the truth.