If you read the following articles you'll see that hit ratios are not a very good indicator of system performance. In some cases very high hit ratios can indicate poor performance due to endless spins for latches.

You can still use hit ratios if it makes you happy, just make sure you follow them up with some real tests or you may find yourself out of a job.

http://www.oracle-base.com/Links/yapp_anjo_kolk.pdf
http://www.oracle-base.com/Links/oraperf_bw.pdf
http://www.hotsos.com/dnloads/1.Mill...CacheRatio.pdf
http://www.hotsos.com/dnloads/1.Mill....11.14-LIO.pdf

Cheers