How to use delta for continuos counter OID for Net Util.

Hello List:

I have searched through forum but cannot deduce clearly how to setup hence this post.

I am trying to setup graph for Netapp Filer's Network Utilization. I have already loaded latest OID through MIBBrowser and see related OIDs which I can probably use (both Low and High)

IfInOctets
IfOutOctets

I am looking at following formula , found in on of the thread in snmp forum:

====================================
For half-duplex media
(delta!ifInOctects + delta!ifOutOctets) x 8 x 100
----------------------------------------------------
(number of seconds in delta!) x ifSpeed

For full-duplex, use the following formula,

(delta!ifInOctects + delta!ifOutOctets) x 8 x 100
----------------------------------------------------
(number of seconds in delta!) x ifSpeed
=====================================

I wish to use this above formula in your OpManager Bulk custom monitor (version 7 build 7102) and have following questions:

1) The OIDs in Netapp I believe gives total Octets since boot time, how to get difference value between earlier reading and current reading and do math on that? If its above formula, does it work this way in OpManager custom monitor? If not how would I setup that. I dont see any other OID for Netapp that give current Ifinpackets/sec

2) I imagine I should add both high and low value to get accurate values? I have already setup to use 64 Bit counter in OpManager device page.

Appreciate your help.

Thanks