I usually do not bother thinking about this rule too much as I only use full Grey Company when playing the Arnor faction, but how does bow limit work when you play 50% warriors and 50% rangers for example? Do you have to field Rangers of the North for the full 50% or only for the 17% you are going over the normal bow limit?

There is no 50%. You either play with the usual 33% limit or their special rule (4 rangers for each ranger of the north/dunedain)

Thanks for your answer. I did not mean having a 50% bow limit in my initial post. What I wanted to ask was whether I would only have to take rangers for the archers exceeding the normal 33% bow limit in a combined force. A question you answered in the second sentence of your reply.