Resource pools and memory reservations can be a wonderful thing. That said, if you have an environment that is constantly changing, then keeping up with these reservations and making sure they are updated can be a real pain. New VMs will spring up with their own reservations defined and then old VMs get decommissioned. If we don't pay attention to all these moving parts, then we may get some unexpected results inside our systems.
The easiest way to stay on top of this is to use the Dynamic Resource Pool Calculator script written by Eric Sloof. This script takes out the tedium and does all the adjustments for you. It will dynamically recalculate and set your memory and CPU reservations on your resource pools. The script can run either from a GUI or from the command line.