Measuring free memory in more efficient way

[namruf15]

Hello, I'm using Thold plugin for various measures. I measure hosts parameters via SNMP connections. Memory consumption has three data fields which I can use for threshold declaration: memory free, memory buffer and memory cached. I want to be able to raise an alert when total sum of these three will drop down to some defined level (sum of mem free, mem buff and mem cached is the real value of free memory which can be allocated for application). Normally I can only set each data field as separate threshold. Is there any way I can do that?

Cacti version: 0.8.8b
Thold version: 0.5