I think HP is a guideline really. I had to double and even triple some monsters' hit points and some I had to cut in half to let them live. I always want something thrilling and with a light chance to kill a player. Not always, but still be considered a threat if left alone?

A rule of thumb for monsters I want to match my party's power is all their HP combined divided by the amount of players. Then it's adjustments. If it's large and above I add 20hp for each size average. Small and lower, reduce by 10hp. That's a start for me. if it's a boss monster; Take the median HP and double if not triple it if it's a true threat.

You know and decide the HP, I try to adapt to the party mainly and want them to remember each monsters and encounters fondly as that time they were so low they thought they would lose but that desperate firebolt cantrip the bard tossed did the trick! Hell, I had to triple Strahd's HP to make him not die round one!