Flytyer, thanks for the Faulkus reference trick. I've also heard people say to hold the lower hand in the armpit of the other arm, grip comfortably with the top arm extended which works out to be about the same.
I've found that a remnant from my old karate days as a teen (can't even remember how to tie that belt knot anymore
) works best for me. In Goju-Ryu, it's taught that your elbow should be one fist's distance from your rib cage for maximum arm strength in a middle block. Since the sweep motion before raising into a d-loop is very similar to a middle block (excluding wrist twist or lack thereof) I've found that to be a very comfortable starting point for me, which puts my upper thumb pretty close to where knowledgable rod builders think it should sit in the wells.
Also wanted to add an observation (as a student myself) that shorter lighter rods will beg for different hand positions and often an intermediate or better caster is best to follow that natural feel although probably not basic / learners. For instance I've noticed the new lighter and shorter rods like the 13' 7/8 Custom or 5120-4 Sage or T&T 1307, etc - sometimes feel better and more natural with the top hand brought down the handle for higher efficiency casting.
I like to start each practice day (any day it stops snowing
) by finding the minimal effort balance of line length, anchor technique, and forward stroke where the least anchor effort produces the most forward power using the least waste possible. Dana and I were chatting and he helped me realize that unlike overhand casting, the ratio of effort between the backcast (d-loop) and forward cast in spey is not equal, it's significantly less if the cast is done efficiently. I guessed 20% to 80% or even 10% to 90% on a switch cast.
I try to get to the "single index finger and thumb" on both upper and lower hands each time out, and often the feedback in my shoulders and arms tells me to move up or down a little, raise the arms a little more or less, etc. Each rod and line has different "total harmony" and once it's really clear that I've come close to it I then progress into the various casts I had on my schedule to practice that morning.
It's taught me to care much less about distance and much more about efficiency, which I am thinking is the more important of the two lately.