First, I am guessing that you're getting this advice from tuning_primer.sh. Every tuning script has its problems. If the advice doesn't make sense, it is not surprising.

You should not focus on the ratio. The problem with ratios is that they obscure magnitude, and magnitude is what you care about. Specifically, the magnitude of misses. But even still, these misses are just numbers -- they don't have any timing information associated with them, so it's doubly meaningless. Who cares how many misses there are, unless a) you know how much time it's taking or b) you know there are more misses than your server can healthily handle?

So here's my advice:

1) Completely ignore the ratio, and ignore all the advice you see on the web about the ratio. The ratio is a surrogate measure[1] that is useless and distracting. Yes, I know 99% of people are going to disagree with me. They are wrong.

2) If your key buffer has free space after the server is fully warmed up, it's big enough. And because it's not allocated to its full size, any "free space" is actually not wasted at all -- the memory isn't reserved, it's still available for anything else to use if needed.

[1] See http://method-r.com/downloads/doc_details/44-thinking-clearl y-about-performance

For a fuller explanation, and a long rant, please see http://www.mysqlperformanceblog.com/2010/02/28/why-you-shoul d-ignore-mysqls-key-cache-hit-ratio/