OctetsIn&OctestsOut

I am collecting two Values via snmp from an F5 BigIP 2400. These SNMP OIDs are for a VIP on the external VLAN. The Values of each are as follows: .1.3.6.1.4.1.3375.1.1.3.2.1.13.10.10.12.19=virtualServerOctetsIn and 1.3.6.1.4.1.3375.1.1.3.2.1.14.10.10.12.19=virtualServerOctetsOut for a paticular VIP (10.10.12.19 in this case) I am able to pull both values via an snmpwalk. Where I am having trouble is making octetsin and out equal a value in Mbps

My goal is calculate how much bandwith is being use (in Mbps) at that polling interval. Then graphing those values to show bandwidth usage per VIP over a period of time. I have created many different equations to try and solve this problem, but none of them have convinced me that the value returned is correct. Can someone give me an example of what to use for a custom OID in OpManager to help collect this information. I have read several of the other posts related to this issue. None of them have helped.