The variable NS-ROOT-MIB:: svcGrpMemberState provides the information about the state of a web server that belongs to a specific predefined group.
So, knowing the group name, it is possible to walk that variable, and determinate if the web server is up (state = 7) or down (state = 1).
svcGrpMemberState is part of serviceGroupMemberTable, and that table is indexed by svcGrpMemberGroupName and svcGrpMemberName.
The OID to walk is the svcGrpMemberState (i.e. .220.127.116.11.4.1.5918.104.22.168.7.1.6) plus the numeric representation of the group name. Then each row should contain the group member name (or the name of the WebServer).
The amount of rows tells us the amount of total web servers on the group; and, the value of each row tell us if the web server is available or not.
If we device the amount of servers up (state = 7) by the total amount of servers, and multiply the result by 100 we can have the percentage availability of the group.
We can compare that number with a predefined value and if the current health is less that the predefined value, the health of the group is not ok and generate an outage.