you might have enough resources to tolerate failures, but you still don't have enough resources to allow VMs to carry on with same performance, that's what that message is trying to say.

if you set that number to 100% then the warning you see will go for sure, but with 0% it's there. if you want to find out ideal number then you might have to try 25% or 50% or 75% etc to see if warning goes away.

when you are calculating available resources, consider total amount of cpu and memory reservation you have done and if 50% resource is lost, then that total reservation is still fully committed resource to corresponding VMs from your remaining available 50%, which then reduces resources for rest of the VMs and they might suffer during their peak utilization and start contending on rest of the available unreserved capacity.

Hope this helps.