calculated item if a subitem is failed

[tybreizh29]

Hello
I have few PDUs I monitor with SNMP, 2 by racks and 8 racks.
On each PDU I get the with SNMP the used power value for each line (x3).
I have some calculated items, the sum off the power per PDU, then per rack, then the total of all racks.
The formulas looks like

pdu.power.total=last(/pdu.power.phase1)+last(/pdu.power.phase2)+last(/pdu.power.phase3)
rack.power.total=last(/pdu1/pdu.power.total)+last(/pdu2/pdu.power.total)
cluster.power.total=last(/rack1/rack.power.total)+last(/rack2/rack.power.total)+...

One PDU is ooo, and of course SNMP fails.
The trick is that the calculated item, fails and on the monitoring I have no idea of the total value (minus the power used by the failing PDU).
I want to have the calculated item even if one or more PDU fails.
I tried few ways :

replace the SNMP request by a script, but then I did not find an easy way to sum that items and there is no key to reuse (it will call my script, each time I want to use the power value of one line of the PDU)
preprocessing failure : i tried to set a value to 0 if the value of the SNMP is on error, but it does not what I want

How could I do my goal?
Thanks for your help
Marc