I've changed Mounted filesystem discovery for Template OS Linux:
Trigger name from:
"Free disk space is less than 20% on volume {#FSNAME}"
to:
Free disk space is less than {$DISK_FREE_PERCENT}% on volume {#FSNAME}

With an expression of:
{Template OS Linux:vfs.fs.size{#FSNAME},pfree.last(0)}<{$DISK_FREE_PERCENT}

And I've added a template macro:
{$DISK_FREE_PERCENT} = 20

I changed the host macro on one system to:
{$DISK_FREE_PERCENT} = 15

I'd like to be able to test the expression on that host by having it fill in the current values (both from vfs.fs.size/pfree and from DISK_FREE_PERCENT).

Currently, I have to manually enter the values (which means calculating the pfree and retrieving the macro value). I'd really like to be able to have zabbix fill them w/ live data...