Hi Iwill,
Those horizontal lines on the underside of a stock unit provide shape for the unit. They are not meant to be markings used to cut down the size of a stock unit. All stock systems are 8" x 10". When you cut the base to the size of your bald area you cut only the back and sides--to any size you want-- but not the front. The front has a graduated hairline and if you cut that you will lose the graduated effect and have and unnatural 'wall of hair.' Here's a diagram to help you.
Regards,
John