Short Shag Perimeters

Cutting hair
Q: In reference to the already asked Questions, about "Box Layers" and "The Lower Layers of a Shag Haircut". My question is: Is there a way to determine a perfect length to cut the sides & back perimeters of a short shag haircut, so that the top (box layers) automatically blend to the perimeter on the back?
I would like my side perimeters possibly 6 inches long, with little teasing and brushed up and back behind my ears. And back perimeter only slightly tapered a little longer than the sides. Box layers as smooth as possible, no choppy or latest style cuts. Top and crown area about 5 inches and bangs little longer as I have high forehead.

A: The easiest way to get the dimensions you want in the hair cut you describe is to cut your initial perimeter guides to the lengths you want in specific areas, and cut the top section of the hair to the length you want for your upper layers.
Then, cut the hair's remaining layers by using thin vertical slices held at 90-degree angles and cut using a vertical cutting line to blend the lengths from the bottom guide length to the one established at the top.
Work your way around the head, working from back to front, first one side and then the other. Overlap your slices a little to keep your layers even, and be sure to cross check the cut when finished by using horizontal combing to check for corners.
See also:
How to cut square layers or box layers
How to blend the back lengths of a long-layered haircut to the sides and front
How to cut a shag