I like this idea. Here's what cropped in my mind almost immediately, based loosely on the Resistance rules from BRP. The basic mechanic is to divide Value A by Value B and multiply by 100 to obtain a percentage chance of Value A defeating Value B. So, using your example of a character with a Courage of 7 (Value A), and a river with a Peril rating of 4 (Value B), crossing it is a no-brainer – (7 / 4) x 100 = 175% of crossing. Bump the Peril rating of the river up to 8, though, and the character’s chance of mustering the Courage to cross drops to 87.5%. Roll the percentile dice to see if he has the Courage. Failure in the resistance roll would result in a specified loss of Courage points.
What if you want a sure-fire chance of keeping your nerve, though? Spend your Courage points. At any given moment, you can opt to spend up to your current Courage score to temporarily add to your current score, but afterwards, your Courage will be depleted by that number. Thus, in the second example above, you could temporarily raise your character’s courage from 7 to 8 by spending one point – you’d have a 100% chance of crossing the river of Peril 8, but afterwards, the emotional exhaustion would reduce your functional Courage score from 7 to 6.
This, of course, leads to a sort of death-spiral, which might actually be fun. Let’s say you want to get back across that same Peril 8 river, but your Courage is now 6. You can either take your chances with the 75% chance of mustering your Courage, or you can spend two points to temporarily bring your Courage up to 8 and a 100% chance. Afterwards, though, your Courage will be further depleted to 4. On the off chance you want to cross the same river again, you’d be at 50%, or you could spend your remaining four points to bring your Courage back up to 8 for a 100% chance, after which your character would be totally emotionally exhausted with a Courage score of 0.
Note, of course, that you could have spent fewer Courage points to merely improve the chances of beating the Peril rating, rather than boosting your character’s chances up to a full 100%, slowing the death-spiral as you see fit. Also note that, with a significant gamble on the player’s part, a character can face down a Peril threat twice his Courage with no chance of failure, but will be a nervous wreck afterward. And it’s also worth noting that, if Courage increases with experience, an experienced adventurer will eventually be able to face down once-dreadful Perils without batting an eye, while becoming increasingly Courageous in meeting more intense Perils.
Personally, I’m beginning to think that I’d like to use a system like this for a more heroic play of Call of Cthulhu, where the Sanity threat is replaced (and re-gauged) with Peril. I enjoy the notion of boosting my Courage to twice its normal level in order to look great Cthulhu himself squarely in the eye...and promptly be eaten. At least it'd be an heroic finish.
!i!