Yes, this is the type of information the DMG could/should have contained. How to modify single monsters for over sized parties. It would be great if it also had information about modifying monsters or encounter guidelines for OP groups as well. Regardless, I think @CapnZapp would approve.Single encounter big bad guys
Add 1 feat/ASI per 4 CR (save the first four)
Add 1 legendary action per PC above 4.
Multiply HP by 1 +0.25 per PC above 4.
Add 1.5 AC (round up) per PC above 4.