CR of a wounded monster?

[gadren]

Has there ever been official rules posted on how much CR should be adjusted if the monster has already lost hit points before the fight? Like, say the party comes across an Umberhulk who is down to half hitpoints, because it'd been in a bad fight a couple days before. It doesn't seem like it should count as a full CR 7...

If there weren't official rules, how would you rule it at your table?

[Totema]

At first this seems pretty simple. In this particular case, to calculate CR I would treat the umber hulk as if it had half its hit dice, four fewer than the base creature. Since aberrations have a +1 CR per four hit dice, removing four hit dice would decrease it's CR to 6.

That still seems rather high though, doesn't it? Especially since it now only has 37 HP? Anyone else want to take a crack at it?

[Cerefel]

aberrations have a +1 CR per four hit dice

Don't forget that increasing monster HD also affects BAB, saves, and feats. Since none of those are happening maybe bring it down 1 CR per 2 HD?

[Renen]

I think you made a mistake here. Since bringing it down 1 per two hit dice would make it's CR lower even though, as you pointed out it doesnt lose things like feats.

[Strigon]

Don't forget that increasing monster HD also affects BAB, saves, and feats. Since none of those are happening maybe bring it down 1 CR per 2 HD?

I'm confused; wouldn't it be more like 1 CR per 6 HD, since we aren't affecting its power level as much?

Under your suggestion, putting an extra 2 HD on a creature without affecting its other stats would pump it up by 1 CR, whereas doing so while also increasing everything else wouldn't.
We would then have a higher-CR creature with lower stats.

Anyway, on topic, I'd say bring it down by (Current HD/Max HD)/2 of its total CR.
So getting rid of half its HD would leave you with (1/2)/2, or 1/4 of its CR to remove, leaving you with a 3/4 CR creature*

*Note, this is just a suggestion. I haven't done it often enough to tell you how balanced it is.

[Cerefel]

I think you made a mistake here. Since bringing it down 1 per two hit dice would make it's CR lower even though, as you pointed out it doesnt lose things like feats.

Oh oops... I don't know what I was thinking when I wrote that