Some other thoughts that might make this skill interesting:

-Reduce energy cost of first aid to 500 instead of 1000. This makes using first aid in battle much more tactically useful, since the PC isn't always risking taking another hit immediately after using it.
-Increase the time that the PC has before wounds can no longer be healed with first aid.

My experience is that damage healed by first aid is proportional to damage taken, more or less.
At high levels, a trap that does 40 hp damage may get 20 hp back on First Aid.
I have even seen low levels of poisoning be more than counteracted by First Aid - does 1 hp damage, recovers 2 hp.

First aid utility is also based on what your current PV is. If a character has 2 PV, every bat and goblin pecks away at your hp. First aid will keep you alive.
With a PV of 20, what gets through is either magic, penetrating, or overwhelming damage.

I have buffed first aid more.