I halve Elite and Solo hp and this works very well. I used to halve Standard monster hp but I found some died too fast, giants especially should not be so squishy. So for Standard monsters I reduce hp as follows:
Low (eg Artillery) 4/Level+4+CON
Medium (eg Soldier, Skirmisher) 5/Level+5+CON
High (Brute) 6/Level+6+CON
This can be done to listed stats as follows, deduct:
This matches closely to PC hit points and I find it works best.