The lower the COI the better, providing there are enough generations to be seen but that also coincides with good health tests of (preferably) both parents? Yes? With the COI of Cocker Spaniels being about 10% then I should aim for under this at least, yeah?
Yes, yes and yes! 😀
Both parents should absolutely be health tested. The only reason someone wouldn't (if they are a breeder) is because they absolutely know there is an issue with the dog they haven't tested and they don't want to not be able to breed that dog.
(Below is a lot of detail, so feel free to ignore)
Chances are you won't get dogs with absolutely perfect health tests. But you just have to decide what issues you could live with (unless you strike gold and manage to find that perfect pup).
For example, PLA (which tests for primary glaucoma) on spaniels I would (assuming all other tests are clear or unaffected and the COI is low) accept a value of 1 (ranks 0-3 where 1 is 'mildly affected' but can still be bred from) from one parent (but not both). Ideally it would be 0 - but getting a puppy where all parents health tests are perfect, the COI is low and the breeder is 'good' is so difficult if not impossible.
However, something like hip dysplasia I would never risk (it's debilitating, painful and with a dog like a cocker spaniel just unfair) and I would expect very low scores.
Ultimately, it is a very individual decision. Some people would go with a higher hip score but demand a '0' on PLA, some people would prioritise a lower COI.