thanks for the replies, I think I understand the reasoning behind using percentage based, but this relies on reservations to calculate slot sizes, and in this case none of my VMs will have reservations set. Therefore HA will utilise the default slot size, which will be a 'false' value, as the number of VMs I can power on will be significantly lower than the number of available slots. Therefore the best way I can guarantee my VMs will successfully restart is to have a dedicated host with equal or more ram than my existing hosts. Think of it as a vertical container for my guaranteed resources rather than striping the resources across the hosts. The key point is I will not have any reservations, if i can quote your HA deep article Duncan
Basic design principle: Be really careful with reservations, if there’s no need to have them on a per VM basis don’t configure them.
IN this case I don't need them so wont be configure them, hence the reason I chose a dedicated failover host