Hey Prj. It's all mathematical. similar to the how the volume calculator works.
Each board shape will have it's own liter unit per cubic volume. (if that's the correct term)
As long as you have a stock starting point which equals (x) amount of volume to go off.
EG. you multiply the length by the width by the thickness then divide it by the volume
hellrazor 6'3 (6.3 x 19.6 x 2.3 = 284.004 then divide it by it's known volume 30.9 = 9.19 litres per cubic volume)
so lets try the 6'1 (6.1 x 19.5 x 2.25 = 267.6375 then divided by 9.19 = 21.9 liters)
My math sucks, but the trick is to accurately convert the inch fractions, computer algorithms do this with a blink of an eye, so it is possible to put an equation like
this into the CDB before the pdf is created, obviously the generated pdf will be spot on. But this works as a close guide with out us lameo's reaching for a calculator