Problem with linux disk space calculation

Hello all,
This my first post to this forum.
Its just 1-2 weeks i am using OpManager for monitoring about 100 servers.
Especilly i am using this tool to monitor the disk spaces of servers. The disk space shown by OpManager for windows servers is fine, but the problem is with the linux servers
I found the disk space calculation for windows and linux servers is done using same SNMP OID. This shows exact free disk space for the windows servers but for the linux servers there is 5% added to the free space.
The problem was with the SNMP OID used. There was not calculation of the 5% disk space used by root.
So while calculating the free disk space we have to consider 5% space used by the system. Thus actual free space would be
(total disk space - used disk space) - 5% of total disk space

Note: I have 5% of total disk space is the default reservation by system which could have been changed to 0%, but for now all of my servers have 5% reservation.

So if i can change the SNMP OID or subtract 5% total space to get the actual free disk space then the reports and alert messages generated by OpManager would not be mileading.

Thanks in Advance
sNkr