thanks for the nice compliments :-)
There is indeed no easy way, SRM doesn't help you with that. Well unless, you would for instance create resource pools and set a reservation on that pool as the minimum your VMs would get. Then map your production to your DR site to that Resource Pool. But it does mean you will need to increase that reservation for your production when the number of VMs increases. But it isn't truly a solution for what you are looking for I guess... It is still all manual math work...