SNMP monitor: sum of some entries of a table

Hi,

I would like to create a monitor on a device which do the mean of SNR values. For these, I have a table (OID: .1.3.6.1.4.1.13732.1.4.4) :
remoteucpTable
remoteucpEntry
ucpnmdid
upsnr
poweradjustment
symboloffset
frequencyoffset
ucpstatstime

upsnr.5026313:10.09
upsnr.5038053:9.55
upsnr.5038276:9.36
upsnr.5296264:9.87
upsnr.5296278:9.95
upsnr.5296300:10.61
upsnr.5296333:9.51
upsnr.5296393:9.87
upsnr.5296496:9.59
upsnr.5296516:10.00
upsnr.5296537:9.53
upsnr.5296553:9.56
upsnr.5296586:9.82

First, when I make a snmp monitor on the upsnr (OID .1.3.6.1.4.1.13732.1.4.4.1.2), I have an error where I have this result thanks the mib browser. Monitors are ok when return values are integer. Is this problem due to the point (for example a return value is 10.09)? How can I get these values?

Secondly, when I will be able to get a upsnr value, I would like to mean values only for some entries (for example, the three first). How can I create a such monitor? Shall i create a kind of script (and where) ?

Thank you