Does this mean that if a guild holds 11 city buffs for instance, their enemies get (11-2)x10% as a buff? (90%)
Correct. And this 90% is added with +5% per camp. So when at F it's 110% boost.
So if you are at F with 65 durability, it's .... 70% power + (90%+20%) total +80% boost.