Custom triggers from RRD data

I installed the XenServer zenpack, and have found it incredibly useful. I've been trying to create a custom trigger within the monitoring template to create an event when the virtual allocation (the amount of disk space that has been issued via thin provisioning) reaches 70% of the actual available physical space. It goes something like this:

Max Value

(here.getRRDValue('property_physicalSize')) * .70

I keep getting the infamous user-supplied python expression for max value caused an error message. I've tried multiple different things, and it seems that it fails only when I introduce the arithmetic into the expression.

Anyone have any idea what's going on