Uggh, nothing about level-appropriate challenges there. So a 2HD critter with 25% magic resistance is nearly untouchable by a 1st or 2nd level wizard. Mean. I think this means we're going to have to fudge around with SR a lot more in these conversions. I guess you've mentioned this before, though.
Well, a 1st-2nd level wizard would have a 70-75% failure rate against a 25% MR, so they'd be advised to throw a few flaming oil flasks or sic their martial allies on the critter.
Anyhow, the assumptions of the game were a bit different back then. A 1st or 2nd level wizard wasn't very likely to meet a creature with Magic Resistance. There weren't that many of them at low level - imps, quasits, pseudodragons and some fey were the usual suspects, and I believe that even those were more commonly met at mid levels.
And yes, I think we've had this conversation about SR conversion before.
I guess we could go a bit lower than CR +9 if you think it's too mean. A 25% MR isn't that special, being at the lower end of the range. In AD&D Imps & Quasits were both 25% MR, Bearded Devils (Barbazu) had 30% and Pseudodragons had 35% MR.
Let's see, In 3E Imps and Quasits don't even have Spell Resistance, barbazu have SR 17 and Pseudodragons 19. That suggests we should be aiming for somewhere about SR 15-16 for a typical Warden Beast (say, one based on a lion, ape or tiger).
That fits pretty well with a Spell Resistance equal to CR +9.