hmmm ... thank you for the reply, but now like many of the controls in vcops, I find myself wondering just what those words mean! 
"First, I note that you have the HA option selected - doing this will automatically use your cluster HA settings for the reserve of CPU and memory capacity (so in your case you are really reserving 50% not 25%)."
We do not use HA admission control (just don't find it that useful) - but I don't understand what "so in your case you are really reserving 50% not 25%" could possibly mean? There are 4 hosts in the cluster. We try to maintain an N+1 capability, which would mean essentially each host has 25-33% excess capacity, so that if one goes down, all the workload can run. Where do you get 50%?
Also you said:
"The reservation levels are, in my opinion, better for making sure that you have a buffer for unplanned growth than for HA."
Huh? Yes, that is what we want - we would like vCOPs to let us know, when our actual useable capacity is close to not being N+1 - we don't expect vCOPs to have any role in actual HA situations, but we want it's capacity forecasting to understand that "N+1" is what we consider "at capacity" so that we need to scale up (or move workload to another cluster) ...
It seems hard to believe that what I am asking for is so difficult, as the notion of N+1 is fundamental to resource planning of a virtual infrastructure ....