Half the reach of Medium is 2-1/2.
Wow, UK certainly has put a lot of time into this project. It's not exactly my cuppa' tea, but I've gotta give him props for all those fractions and equations.
Yeah... he said he spent something like 6 months coming up with all that. I find it invaluable - once you get the hang of it, you can figure up a creature's CR in a few minutes (things with a lot of SLAs take a little longer, of course...).
Ugh, there are so many details that irk me in the D&D rules, I think I'd end up just rewriting the MM. Anyway, are you going alphabetically or by type or what? And are you doing monsters from splat books too?
That's pretty much what I'm doing - fixing all the stupid/nonsensical stuff along with converting. I'm just working my way through alphabetically, because that's how the SRD files are organized. As for splat books... I'm not sure if I can post conversions for non-OGC material or not (even just "Change AC to xxx"), but since I don't have any splats, it's kind of a moot point.
Even if you change the size modifier to 0, big monsters' extra strength and extra hit-dice-fueled BAB still leaves them almost out of reach. In the worst case, a Gargantuan black dragon has a +46 grapple check without the size modifier. That's why I house ruled that
the defender doesn't get to add his BAB to grapple checks. It makes escaping just tough enough and scales great for giant monsters.
True... but if you use the rule that you can't grapple something more than two sizes larger, you don't have to worry about it as much. That made me think, though... the Snatch feat enables Huge or larger creatures to grab someone. You'd be grappling against their hand/tentacle/claw/whatever, not the whole beast, so the size modifier should be smaller. That would probably be best inserted as an optional rule, though.