Hm. Maybe I did, but I was probably in a hurry somewhere.
Anyway, that particular way is inefficient, especially since it's unnecessary to define individual percentages. That's only a part of the system I'm thinking of, btw. I wanted to include attack size, and attack patterns.
Attack size is Just What It Says On The Tin - the size of the attacking object. Since we have overall body size and relative part sizes, it's not too difficult to see if an certain attack will be larger than a certain part. Think a hammer strike against a rat. A rat's size (under the new system) will be around 1800-2000. Its body will be around 1000-1200 of that. If you hit the rat with a maul with an attack size of 1500, and hit the body, you will have the 300-500 propagate away at the neighboring parts, equally - to all limbs, the tail, and the head. The proportional reduction in attack size will lessen the damage dealt as well. On the other hand, think a dragon against a ballista bolt. The dragon's size, if we assume its size under the new system to be 20000, is sufficiently big, quite enough to make any attack affect only one part. However, a ballista bolt is large, somewhere in the 4-5k region. Hitting the main body will hardly propagate the damage further, but hitting the foot will likely cause damage to the leg as well, and hitting the upper arm will likely shed off some damage to the body.
Now, attack patterns. These are fairly simple. First you have the "COMBO:X", that chooses several parts at random and hits them, as would be if you're pounced on by something large or hit by, say, several tentacles. And combo attacks, too. Then there's "CHAIN:X", that chooses a starting part and goes from it along the body tree, attempting to hit parts consecutively, used for slashing attacks. And finally the "SINGLE:X", a variant of the combo that attacks the same part several times. Added to the patterns is the RANGE, which works similar to the size but affects not the damage propagation but the target choice for multiple-selection attacks - so that you can't slice a dragon in half with one stroke of a shortsword. Of course, the attack size still works for multiple parts hit.