We don't usually get to pick where the power vent hole is in relation to the lines the shingles or slate are
laid by. If I had my druthers, the vent hole on the example roof would have been about 2=3" lower or higher on the roof.

That said, roofers are supposed to use the 50% rule of thumb for handling penetrations. This means that the material underlying the hole flashing should extend at least 1/2way past the penetration before installing the flashing. A lot of roofer prefer to 'hide' as much flashing as possible. Sometimes all of it. It can be done, but it's not a natural way of doing things because the methods used create hindrances to the natural flow of the water. These hindrances often lead to leaks.

In this instance, the number was about 75%. The alternative was to have run 1 less shingle under the flashing which would have resulted in 25-30% under the metal. Unacceptable. In this example, the flashing extended below the underlying shingle about 2" and onto the next shingle below it. This created a 'head/termination flashing' length of 5" below the lowest point of the hole. Purely a mater of aesthetics.

( If you ask any roofer what the standard is for termination/head flashing, the answer you most often hear is "5inches".)

The reason for going under the flashing X number of inches is to catch any traces of water that may run laterally over the side of the flashing in a wind driven rainstorm.

Running a course only 25-30% under the flashing would allow any wind driven water to land on the felt, where the shingle would then divert the water sideways across the roof and show up somewhere as a leak. Usually appearing below the vent.
Doing it with only 20-40% under the flashing will require sealants and possibly Ice and Water shield under the vent and over the lower shingle, plus a course over the edges of flashing to insure watertight integrity. A lot of extra work to achieve the same result as 50-70% underlying shingle gives you.

Though blurry, you can tell in the next picture that water blown laterally would go under the leading edge of the flashing, but would have to travel about 8-9" to even reach the bottom of the hole. Not very likely with dimensional shingles. Soon as the water hit the first irregularity in the shingle, it would then flow downhill.

I next installed the first overlaying course of shingles leaving about an inch of clearance around the vent itself to help the water freely flow down and past the penetration. This is important! You don't want to hinder downward water-flow in any way. Doing so gives the water a chance to find some flaw in the install and create mischief. Namely, a leak. Notice the shingle tops are cut square instead of rounding the vent. Any windblown water will hit the squared edge and be directed down the roof, not across the roof.

Then next course was cut to follow the curve of the vent and about an inch of clearance was left around the top to further promote the smooth downward flow of the water.

Take note of the abbreviated nail pattern to avoid putting any holes or unnecessary nails in the vent flashing/base.

As the finished product shows, there are no built-in hindrances to fast, efficient, natural water flow.

The only things left to do are run the last courses of shingles and re-install the top of the power vent.

Now, here we have another picture of one done the other way, with only 1 shingle under the flange.

( Photo courtesy of "TheRoofingGod" )

In this scenario, the last shingle under the flashing  barely makes it to the hole by about 3". That's only about 15%. The top of the first shingle over the flange is actually below the halfway point of the hole.. The top of the second shingle above the flashing barely makes it to the top of the hole. We now have 2 'tips' of shingles that can catch wind-driven water. If the installer took a lot of care in tapering those edges away from the vent, it may not be an issue. If any water does get misdirected under there, the roofing felt will be the only line of defense. The water cold reach the felt, hit the top of either overlaying shingle and travel sideways to the vent opening, or even across the roof elsewhere.

Here' s one that had no shingles run under the flashing of either vent. The installers came back and cemented all the surrounding shingle keyways in an attempt to stop the leak. They didn't even have a clue when they first messed it up. Yes, I was looking at it because it was still leaking.