I dont do this for vms but i do for datastores. The way i do it is separate alerts
One for normal sized and one for lard datastores.
i create sub policy off my main policy for each group i want to create and disable the normal alerts for the datastore in them and enable the new alerts for the different groups in each sub policy.
I then custom groups with membership rules for greater than 10tb and less than 20tb and a second for 20.1 to 30 and so on and assingn the correct policy to that group that has the alerts enabled in them
I hope this makes sense
I know it does not answer your question but may be an alternative
Thanks sxnxr! That does make sense and will certainly solve this problem.