When you walk through the soot, you can get from 5 to 15 grams (which is common) or 20 to 100 grams (which is rare), when you discover an item under the soot, you get 3 to 4 grams, and when there's a Ninja Kid under the soot, you get no soot.
When you talk to the Soot Man he'll give you a list of items that he can make out of soot
The minimum you need is 200 grams, and the maximum amount you need to get the Elegant desk (which requires the most soot) is 8000 grams.
There's no real way of finding out the actual amount of soot you have, but you can assume how much you have by what the Soot Man tells you (whether you have enough for an item or not)
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