VM resource utilization really depends on several factors. According to your logic (which is not wrong, but very simplistic) you can user 100% of resources of a specific host (the resource is there to be used, there is no reason to waste X% of resources "just in case") in case of HA failure - you will be recovering the lost host ASAP, I imagine.
HA reserved resources are calculated in a per-slot basis. The KB http://kb.vmware.com/kb/1010594 demostrates how this is performed on the vCenter. Having this in mind, you can adapt/calculate slots for your systems, and then know what will happen in case something fails on your cluster.
Hope this helps.