How to find the hit ratio percentage

Hit ratios below 90-95% are usually a sign of poor indexing, BUT Distortion of the hit ratio numbers is possible

select (1-(sum(decode(name, 'physical reads', value,0))/
(sum(decode(name, 'db block gets', value,0)) +
(sum(decode(name, 'consistent gets', value,0)))))) * 100
"Read Hit Ratio"
from v$sysstat;