It's hard to figure out, the SRD doesn't tell you much about adding the swallow whole ability, only to look at the creature to determine how much damage you take. Most creatures with swallow whole do 3d8+str and 2d6 + half str damage and some acid damage, but they're also all magical beasts. I guesstimated that I needed to drop the bite damage by two categories and do half strength. Since I wanted a normal animal and not a magical beast I only did 1 point of acid damage since it seems to be a slower process normally. I ended up with 1d4+2 damage from the muscle action and 1 point of acid damage a round. I figure that if the creature dies, the muscle damage stops and only the acid would continue until you get out.