DMG talks about it in monster creation, but you can also check it against any of the monsters in the Manual, first monster I thought of, that's easy to see, Balor https://roll20.net/compendium/dnd5e/Balor#content
+6 to Con, 21d12 hit dice. So that's 21*6.5=136.5, plus the 21*6=126. 136.5+126=262.5. Hit Points listed for Balor are, 262.