Calculating your Offensive/Defensive PF Rank in The Phantom Pain

Can anyone tell me how the Offensive/Defensive/Durability ratings for your Private Force Rank battles are calculated? I've been examining them and I've made some progress, but I'm still a long way away, and there's no explanation in the (otherwise) amazing official guide.

Consider my Offensive Capability/Durability grades:

"Lv. 74" refers to the level of my Combat Unit. The "385" refers to the number of people in that unit. The "Total Development Grade" refers to the items I've developed (the more, the higher the number).

And my Defensive Capability/Durability grades:

"Lv. 88" refers to my Security Unit level. "375" is the number of staff in that unit. "Total Defense Level" refers to all the levels of my different FOB struts added together.

But how are the main numbers calculated?

Turns out the official website (which I wouldn't have thought to look on) has the answer:

I don't have an exact formula, when you highlight a PF League "combat", you can press T (on PC) to view the details of the combat. This screen tells you exactly where all the point are coming from (number of combat vehicles, staff with the Medic3 skill, parasites,...)