Percent free space is .1.3.6.1.4.1.789.1.5.4.1.6
Absolute free space is a combination of two OIDs, as NetApp does not seem to have discovered 64 bit counters.
.1.3.6.1.4.1.789.1.5.4.1.19 (Low Available bytes) and .1.3.6.1.4.1.789.1.5.4.1.18 (High Available Bytes)
And more aggravatingly, NetApp returns these as signed integers, when they are not.
So you have to correct to get the actual value, depending on whether the first bit is set or not.
So to get Free Space:
((if(lt(dfLowAvailKBytes,0),4294967296+dfLowAvailKBytes,dfLowAvailKBytes)+(dfHighAvailKBytes*4294967296))*1024)
And another issue - the indexes of Volumes change fairly regularly, if you add/remove volumes.
To avoid all these hassles, we use LogicMonitor.com which automates all the volume discovery, the index changes, the space calculations, and gives latency per volume and other stuff not available through SNMP.