Agree with other poster about summation values. You can see this easily by looking at realtime chart advanced for CPU, using chart properties to only select CPU Ready, then exporting to CSV. You'll see that the total VM CPU ready is a sum of all the CPUs on the box. Additionally, this will explain why you see reduced CPU ready when removing processors.
Further info in case you are interested, from my experience.
High %Ready means that the VM was wanting to execute on a slot, but no slots are free. Generally this host is too busy or there are CPU limits in place.
High %Costop means that the VM is wanting to execute, slots are free and one or more vCPUs are in line to use it, but not enough free slots to satisfy all the processors in the VM. While this can be caused by host too busy, I see this more in environments with too many multiprocessor VMs running on the same host. Although host too busy can cause costop issues as well.
So if you have a ton of 1 vCPU VMs bursting at the same time, you can have high CPU ready but will have 0% costop. This is still indicating a performance problem (host is overloaded).
Usually hosts have a little of both kinds of VMs, small and large from a vCPU count. Your hosts have 24 cores, 48 slots with HT. I would be surprised to see costop from an 8 proc VM unless you have a lot of other high proc VMs on this box. It is hard to know without taking a larger look at the environment.
All that said, I think you might be overthinking this a little. You don't really right size individual VM CPU using CPU ready, you right size it from the processes running on the guest. You say "you removed 2 processors," but what is the utilization in the guest? Generally you start small, and grow if the guest hits 100%.
CPU ready tends to be more of a metric for host loading or a measure of the VMs as a whole, not individual guest sizing. CPU for a VM is "right sized" when it is just large enough to support the workload (or sized to support workload scale). A VM with 8 processors that is using 1% of them is not right sized. And a VM with 2 processors that is using 100% of them all the time is also not right sized. So if you look at your VM with 6 processors and it is using 20% CPU, it probably needs to come down more irrespective of the CPU ready metric.
Hope that helps.