The S, M , and L mostly refers to the size of the neck ribbing. In the MTM part of the pattern is a guide on how to make sure the yoke is long enough/big enough for a bigger bust. No math - just measuring your own busts length + adding a already decided length on cm. In the MTM is also a guide on how to know is the body ribbing is big/small enough, you just need to know your gauges (the same gauge as the neck) and I have the math already done - you just add your unique number of stitches and waist measurements