Make it consider how much health is missing rather than how much health there is in absolute. The way it is right now is (as in the title) quite ridiculous.
I don't think the current system is ridiculous. It's just not always what the player would want! I actually like it the way it is. The current system has the advantage of being easy to understand (ie. it's easy to work out who will receive the healing, without having to get out a calculator), and it makes the heal more situation dependent. What you are suggesting would make it more powerful; more difficult to describe; and, in my opinion, less interesting.
Help The Weak is considered a lower quality card, which is why it has a bronze border. This is also what makes it available on so many lower level items. Considering that it it heals for 5, has no range limit, and doesn't require line of sight, it's only real limitation is the behavior you describe where you can't pick your target. Heal is considered a better card, even though it heals for less and requires line of sight. The only real place you don't want to use Help the Weak is when you're running characters of vastly different hit point totals. In this case stick to the other heal effects, otherwise it's a great card.
I think Help The Weak is possibly the best bronze card in the game. It definitely doesn't need buffing .
