The NPC/Item events had a terrible bug in the last release, please update to version 0.682 if you haven't already.
The spell code is being worked on right now, and the new code is not enabled in the release yet. The new spell dialog includes a flag to indicate that the players cannot memorize a spell. Such spells would still be usable by NPCs/Monsters/Items.
I will add more options to the item editor to allow restricting item usage to x times per hour/day.
There is an event type called Logic Block that is almost ready to do what you need. Currently, it can only query some character attributes. My intent is to let it set those attributes as well.
The Logic Block is powerful, but also a little more trouble to use. It has a series of configurable inputs and operations on those inputs. Based on a final result, different actions can be taken. There is some detailed info about it in the help file.
One of the input types is called RunTimeIf, for run time interface. A list of runtime variables can be browsed to select the attribute to be used. For instance, $CHAR_AC will put the characters armor class into that input field at run time. You can then apply various math operations on it, even in combination with other attributes. The problem is that character attributes cannot be set yet, and the character who provides the attribute values cannot be set yet. Once this is working (soon), you will be able create item events that modify hitpoints. I just completed adding 71 character attributes to the runtime interface, so that part at least will be in the next release.
The other issue is about automatically reducing hitpoints to simulate fatigue/hunger. Something I have thinking about adding is events that trigger based on game time. You could set one up to trigger every x minutes, using a Logic Block to reduce hitpoints (or THACO, AC, etc). The player would need to use food/water items to restore, or also spells/resting.
It is easy to rig things in a fixed manner as you suggest (x+(con/2)), but I would like to stay as flexible as possible. Some designers may want a different formula. At any rate, I'll have to think about it for a bit to consider the options.
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CocoaSpud
Dungeon Craft Development Team
http://uaf.sourceforge.net