Wondering about the data.

I would assume that as well. Lately, I've heard some people saying there's a "new" heal hack out abusing this:

 NB, contains image of a criminal

monster graphics are stored client sided. There's no way an MS server would need those graphics.

That's not a new hack. That thing has been around for a while, and personally, I think it's one of the most useless hacks in the game, next to fame hacking. The best places you could get exp using heal are on the monsters that are already heal weak. Other places, even if you change the monster's data to heal weak, are either too strong (such as Wolf Spiders, a popular place this is used at), or not mobbed up enough. For example, using it on a wolf spider would require 30+ hits to kill a mob of six, which is utterly pointless.

How would you store a fraction, though? As a real decimal approximation?

Yes. The reason I say this is because sometimes the damage doesn't correspond to the damage the monster is actually taking.

For example, a while ago, there was a glitch in which it took a 16x at least 4 hits to kill a snail, and 20 to kill a newtie. I think there are still some vids on YouTube about it. The only way that glitch could have happened is if the decimal approximations were off. If the direct damage was sent to the server for processing, that glitch could never have occurred.

The server reads those stats. It's moreso for them than for us.

So does the server store receive the damage data as a decimal percentage or a direct number?