i have a question about applying the carson rule for frequency modulation.

So i know that in frequency modulation, there is the carrier signal and then an infinite number of sidebands as well. If i'm not mistaken, the carson rule can give a bandwidth which shows where 98% of the power spectrum exists, thus, showing the majority of the "useful" side bands. The problem i am having is how to calculate this.

i know the rule is given by

CBR = 2(Δf+f_m)

i think f_m denotes the carrier frequency, but im not sure how to get the value of Δf. I've read that it represents the "maximum instantaneous difference between the modulated frequency and the carrier frequency", so does that mean i take my higher sideband location and then subtract my carrier? im getting some numbers that don't really make sense, so i'm not sure if i am comprehending the equation very well.

TIA