Oracle DBA Script : Calculate buffer cache hit ratio in DB

Calculate buffer cache hit ratio in the database. Make sure it is more than 80 for an oltp environment and 99 is the best value.

Code:
column "logical_reads" format 99,999,999,999
column "phys_reads" format 999,999,999
column "phy_writes" format 999,999,999
select a.value + b.value "logical_reads",
c.value "phys_reads",
round(100 * ((a.value+b.value)-c.value) /
(a.value+b.value))
"BUFFER HIT RATIO"
from v$sysstat a, v$sysstat b, v$sysstat c
where
a.statistic# = 38
and
b.statistic# = 39
and
c.statistic# = 40;