I think somewhere around 50 hp is good. Just seems like a nice number. Allows for enough hp to absorb common AOE and still have a little bit to duke it out with something. Good for both PCs and opposing creatures. Around 30 is where most spells or a deadly opponent can dispose of you in a single turn with a bit of luck. 50 has a bit more staying power and is appropriate from level 5 to about level 11 or 13 (increase numbers as appropriate, actual results may vary with PC progression and/or opponent selection).
On the note of health progression: I like the formula being double the con score added to rolling the class die without adding con modifier. If con is reduced, every point reduces HP by 2 points. Has some odd math stuff where 0 con results in still having HP, but not that hard to compensate. To make things different I added a modifier to monster size, so size small is just con + rolls, size large is 3xcon + rolls, etc. So far it seems to do a really good job of making an opponent easier to judge before getting into a fight and makes low level threats even more credible through the entire game, but the difference in damage and things one can do changes with an increase in level. I did more house rules onto of this system, but I find it neat how the premise of lowing high level hp and boost the lower level hp was viable enough for me to invest a lot of time going through it. Now I can make encounters with orcs that are credible threats, but still evaporate in a satisfying way.