Italyhound wrote:If I get all the coffee in the cup with my bottomless PF, I consider it perfect.

I only use a bottomless PF on my NS Oscar these days. Getting that really helped me tune in my shots, and in the beginning I too had all kinds of spritz, sprays and mess to clean up. I thought my Baratza Vario was clump free enough, that my distribution was good enough and that my tamping was good enough, but the naked portafilter showed me all kinds of issues with channeling.

A friend suggested I give WDT a try with a paperclip. I was dubious, but did it anyway and my channeling was an issue of the past. Now who wants to do WDT every shot? Not me, so I made a WDT wisk and now quickly wisk my espresso grinds in the basket before taking off the basket funnel. Just a couple swirls, starting at the bottom and ending on top of the grinds so they are all even, then a light tamp, remove funnel, finish tamp, lock, load, pull. Probably takes me 3-5 seconds per load to wisk like this and I don't have spritzing at all ever. Extracts that are very even.

Finding the blonding point is easier with a spouted portafilter as Jim suggested. You can clearly see when the stream goes translucent because you will start to see the reflection of the spout through the stream.

It looks to me like it's doing a bit of a donut extraction, and that would be why you might be tempted to cut it off early. That said, if I'd poured it, I'd steam up some milk and drink a cappuccino. What TomC said is the real key - how did it taste?

As to your question about the ratio of good to bad pours, my rate is generally increasing with time. As my equipment and skills to use it improves, so do my shots in consistently hitting that good-great level. The way I rate shots is like this:

Poor: Can't drink it, even with milk.
Fair: Don't want to drink it straight, but it's fine as a cappuccino.
Good: Straight espresso. Tastes good, but doesn't knock your socks off. Makes a great cappuccino.
Great: Drink it straight and then for the next half hour or so your taste buds can still taste that liquid gold.

In the beginning (first few months), I used to get about 50% poor shots. Currently it's down to under 5%. I actually have to try to make them that bad. Lately, most of my shots are Good, about 20% are Fair (usually when dialing in a grind). Now the thing is, this is all entirely subjective. What tastes Great to me, might be Fair to you or vice versa. My wife still gets nostalgic about the old steam toy and blade grinder days when I made dish soap foamed milk and served it to her in a travel mug (she still likes her 'cappuccinos' in a 12 oz coffee mug). But she doesn't really like coffee that much anyway.

I think with your K10 you shouldn't need WDT, I know with my Pharos I don't. You don't happen to bang the portafilter after you tamp it do you? That would create some side channeling and the subtle donut extraction visible in your video.