/cgl/ - Cosplay & EGL

how do i calculate the length go my fabric needed for ruby's skirt? there's 10 pleats total in the skirt I figure the pleats need to be 2.5" long and have a 2.5" gap between each of them, and 2" seam allowance, so would I do (my waist x 5)+10(2.5)+2?
i feel like thats way too big.....or does the total length of each pleat/gap have to equal exactly my waist? so 10(waist/5)+10(waist/10)?
i'm utterly confused since ive need done box pleats like this, especially since i'm trying to get it puffy