Service availability calculation issue

[kirill.login]

Hello.
I have several instances of the web service balanced with round robin.
Each one of them has JMX MBean that represents it's params. Each MBean has a attribute, let call it Health, that represents availability of the whole service.

I wish to get a summarized mertic that represents an availability of the all service instances. So I have create a Calculated item with the following formula:

(
last("Health1") + last("Health2") + last("Health3") + last("Health4")
) * 100/4

Result value can be 0%, 25%, 50%, 75%, or 100% in dependence of count of the "alive" services.
It works well until the application server get restarted for maintenance.
At this moment JMX host becoming off and value of the Heatlh item stucking in last successfully readed value. Item itself is becoming to NOT SUPPORTED state, not to 0.
Calculated item keeps showing me 100%.

So question is: what is the smart way to calculate summary availability with JMX items?

[splitek]

Can you build some item that will check if app server is on or off? And, let say, return 0 if serwer is off, and 1 if on? If so... you can add to formula something like:
* last("serverstate")