Display computed values - how to

for disk monitoring I need to display computed value.
There are 2 scenarios:
1-) From snmp check I have value in blocks. I want to display in (some) bytes. Then the snmp value must be multiplied by 4096
2.) From snmp I have two values, one for total value, second for used. For displaing used space in percent I must (used/total)*100.
How to configure Host. Service and Command objects, please?