If you calculate the carbs (or calories if that's what you want) for a whole recipe this knife guide will help you cut accurate slices you will know the carb counts for. It has separate sides for 8" and 9" cake pans. Hold the correct (or closest) side up to the plate, then make cuts at the slice size marker you want and the mark with the pan size indicator. Note that the picture shows the 9" side being used to make a 1/10th slice going left-to-right while the 8" side goes right to left. Measures slices from 1/16 to 1/6 of the whole cake.

Needs a colorswap after the baseplate to make the numbers and knife guides legible.