We are using other products which get 2 or more snmp values and then calculate the difference like (used disk space / max disk space) * 100%
in this way we can see the % free diskspace. We also use this calculation on UPS to calculate the remaining runtime in %.
We plan to implement some external utility that will be able to process arithmetical expressions like OID1 + OID2*OID3 but I am not sure when it can be implemented