The math is very wonky. I clear my assignments, and then I have 2.55e20 particles unassigned. I then go to assign 1.0e20 to auto sparks, and click the [+], but it assigns 2.55e20 instead of the amount I entered. This makes it very difficult to split assignments between sparks and compressed sparks the way I want.
Also note that this is not the only place the numbers and math are wonky. Wood/Total Wood/Burnt Wood, etc, is another that is.