- make a chain of changes (-7 -3 +5 -1 - 4 ...) and calculate the resulting number at irregular intervals
I suppose if the monster is getting healing or temp hp regularly, this is okay, but in general I think it's a terrible idea. Here's why.
I've seen several DMs track monster hit points by writing down the damage the monster took (express as either a negative or a positive number), intending to add up / subtract down the damage
en mass at some point. It never works, and it takes longer than just adding / subtracting as you go
The DM will be staring at a list of numbers like this and trying to determine if the monster is bloodied or dead:
23
17
8
-5
35
6
I honestly don't understand why anyone would ever track hit points that way.
- keep tally with poker chips
That's what I do for my PCs. In 4e D&D, I started using red chips to indicate the bloodied half of my hit points, and white chips to indicate the unbloodied half. (Instead of the typical convention that red = 5 points.)
With low level PCs, I create one big stack of poker chips. Say, 12 red and 12 white for a 1st level PC with 24 hp. As the PC takes damage, I remove white chips from the stack. When I get down to the red part of the stack, the PC is, obviously, bloodied. It's a nice visual representation.
I use blue chips for temporary hit points.
With high level PCs, the stack gets too tall, so I have the white chip stack in front of (towards the rest of the players) the red chip stack. For example, a PC with 90 hp would have a white stack of 45 in front of a red stack of 45.