Here is something I’ve been working on. Try this pattern out… I’m still working on the ascending version.
Practice a one-finger-per-fret setup first, then move onto the crazy Shawn Lane ideas. I’d recommend spending 90% of your practice time with non-stretch intervals as a general rule no matter what technique you’re performing, and then apply the ease of that technique to the stretch intervals. The reason being that stretching the left hand involves massive tension, which will seep into your playing and degrade your technique. If massive left-hand stretching is all you do, you can permanently injure yourself, and, short of that, make your normal playing sound labored and stilted.
The human hand in every position on the instrument has a preferred relaxed posture. When you push your fingers down, the spacing of your fingers does not change itself. However, the fret spacing decreases as you move up the neck, meaning even with the same relaxed fingers pushing down, you might get different notes. This is contingent upon scale length, among other things.
For me, this starts around the 16th fret. Everything from 1-15 is one-finger-per-fret. Around the 16th, I start getting wider stretches even though my fingers are still spaced the same. If this is confusing, I can make a video and show you what I mean. It’s a nebulous concept and it’s not something I ever see discussed on guitar forums.
To put it simply: Stop trying to cram and/or stretch your fingers into a fret spacing. This causes massive tension. Just pick a position and see where your fingers naturally gravitate to as you floor it with a three-note-per-string combination (1-2-3, 1-2-4, or 1-3-4). The main takeaway: Stretching or cramming is not at all natural for the human hand on this instrument, and it was a huge technical barrier for me. I started learning twice as fast one I obeyed the natural inclinations of the body. You can deal with stretching after the fact. See Doug Steele’s thoughts here:
Shawn Lane talks about the idea of making it easy at first, as does Guthrie Govan. See below video for Guthrie’s advice. I can’t find the time-stamp for where Shawn Lane recommends making the patterns simple at first. It’s in one of his instructional videos.
tl;dr: Avoid stretching or cramming when you are practicing. Yes, music does require stretches or cramming sometimes, but when you’re in the practice room, I really think you should make the mechanics as easy as possible to avoid injury. You can then apply these general ideas to more “insane” patterns with the confidence of knowing you have the basics down pat.