How do instrument modifiers work?

The data field has always shown 24 internally. To prettify the math, many people referred to this as a 2.4

modifier (multiplier) over what you’d do if you didn’t have the instrument equipped, which we take as a baseline of 1 or 100%. This is equivalent to adding a modifier of 140% to what you’d do if you didn’t have the instrument equipped, i.e., 2.4 is 240% of 1, or ((1 x 100%) + 140% [what we show]) of 1.

So in other words, if you had a 140% percussion modifier the 11 damage on furfils chant would do 26 damage with percussion modifier.