You need more captains than FOs.

Generally speaking, CAs are more senior. This means they have more vacation time, and you need more of them to cover for that. CAs tend to be the pilots in the training dept, CAs tend to be pilots involved in management. CAs (and this is very general and doesn't totally apply to our airline) tend to be older and more subject to FMLA leaves. There are several individually small reasons that add up to why you need more CA than FO, I don't know what the number is, I just know that your method for figuring out the bypass is flawed.