Shingling detail for hip and ridge
No problem with how the main shingle run is cut back. I think his idea is that it make 3 places that ease the cap over the ridge rather that trying to do a "hard" corner. If the temps are cooler where you are the ridge cap shingles may crack over a corner that is abrupt.

You are correct about the chalk line, use blue chalk or another non-permanent color, that washes off with the next rain. Do not take for granted that a 12" wide cap will finish off so a chalk line at the 6" mark will be even. It most likely will not. Try 2 shingles with no nails in place to get where the chalk line really needs to be. I ask you use 2 shingles because 1 shingle all by itself with no line to go by can go crooked without realizing it and if you set your line by that, the caps may be off centered. Two shingles help to square each other in a little easier.

When cutting the cap pieces unless you have specified cap shingles already on hand, (some makers provide precut cap shingles) and this also depends on if you have 3 tab or archtectural shingles, the kind that have extra pieces added to them to make them look ore rustic. The latter shingle I think hast the precut caps. If you have 3 tab shingles, typically the caps are cut from scraps and left over shingles. The trick I do with this type of shingle is angle the top part of the shingle that is covered by the next shingle to be laid about 1" or so back at an angle so the upper edge of the shingle is hidden by the next one. It makes a cleaner install.