Changing condition for Critical & Warning Thresholds

I've decided to watch our Exchange Servers by specifically adding the Free System Page Table Entries counter into APM.  I've got quite a bunch of experience acting as a script kiddie with WMI Scripting and VBScript, so I went that route for pulling the data.

I'm trying to change the warning and critical thresholds though.  This statistic is supposed to be large (big is good here).  How do I change the warning and critical thresholds (or my code if absolutly necessary) to handle alerting on a statistic as it falls and not increases?

My code follows:

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strComputer = wscript.arguments(0)
strUser = wscript.arguments(1)
strPassword = wscript.arguments(2)

Set objSWbemLocator = CreateObject("WbemScripting.SWbemLocator")
Set objWMIService = objSWbemLocator.ConnectServer (strComputer, "root\cimv2", strUser, strPassword)

Set colFSTE = objWMIService.ExecQuery("Select * from Win32_PerfFormattedData_PerfOS_Memory")

For Each objFSTE in colFSTE
    intValue = objFSTE.FreeSystemPageTableEntries
    WScript.Echo "Statistic:" & intValue
Next

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