It’s not even that simple. A while ago I did some testing (in keep, and only on dummies, so not fully reliable), and the results were interesting. Here’s the relevant post. In short, it seems that in a certain range band between a bullseye hit and the normal outer edge for Fire Walk, the game for some reason applies what seem like extra ticks of burn with possibly slightly different rate from normal. They’re still apparently considered a single DoT, though, which results in a huge tickrate, especially for Lingering.
In my experiments, the range added by Kaboom! only put out the normal ticks (iirc), but the results may vary. I’d be interested to see footage of testing that from a couple of different people, both to ascertain whether my tests were correct and whether conditions change the outcome. If these results are correct, it indicates buggy behavior for Fire Walk.