Thanks again for the replies, nothing like a little HA admission control to get the brain buzzing, and kicking off a heated internal debate 
I understand the reasoning behind the percentage based admission control, but I took into account the fact that reservations will not be set on any VMs and the fact that a dedicated host, for me anyway, is the best solution with little additional management or consideration when adding additional hosts, as Duncan mentions in his HA article ..What you see it what you get.
I just have to live with the fact that during maintenance periods I'll have to disable HA,and manually move VMs over to the 'spare' host..
Thanks again fro the information