Did someone mean that RATIOS are absolete now and those should not be considered now.
I'll recomend Optimizing Oracle Performance by Cary Millsap and Jeff Holt.
http://www.amazon.com/exec/obidos/tg/detail/-/059600527X/qid=1126645307/sr=8-1/ref=pd_bbs_1/102-8940539-9454540?v=glance&s=books&n=507846
You'll find the answer of your question there.
Cheers, Dimitre Radoulov