Just watched EC's video on chasing the lands.
I come from a high performance engine mindset where the answer is always "measure it".
Now I get it.
TLDW
assuming mag length doesnt keep you off the lands anyway
Find Jam(seat a long(long enough to force the bullet to seat deeper when chambered) dummy round and chamber)
remove & measure
-.020 as starting point
load -.020, -.023 -.026 -.029 etc
shoot til you find at least 2 consecutive nodes
this gives you a .006 range for seating depth
Load to that max range -.001(so if the range of the nodes is 2.155-2.149, load to 2.154)
and thats your seating depth
Then if that stops working, load a little longer such as 2.157 and maybe 2.160 and try those.
So its not so much about a measurement, but finding a place where it shoots.
Cortina is obviously a world class shooter. I am obviously not. But I've been reloading for over 20 years and have chased a few fads and tried a bunch of "conventional wisdom" in that time. I won't sit here and argue that .003" seating depth changes don't matter in benchrest and F-Class world. I don't compete in those worlds and don't shoot those highly tuned systems. But I've never been able to make it repeat in my rifles with the bullets I shoot. So, I will challenge you to prove (to yourself---you don't owe me anything and I don't really care what you do) that Cortina's seating depth "node" approach works in your rifle system. Do it. Select a seating depth using his method, then load 10 rounds. But then test it: pick a random (but safe) seating depth (0.20" or 0.50" away, or the "worst" one you observed using Cortina's method) and load 10 more rounds. Shoot those two loads on the same day (10 shots at same POA, so a different POA for each load, but let it cool however much you want) and see if you can tell any difference in the group size. When I have tried that, I don't see a real difference. I only shoot factory rifles; YMMV.
As another point of reference, when I wrote in to ask Barnes for some load data one time their email back recommended starting their solid copper bullets at least 0.050" the lands and trying different seating depths in 0.025" increments. Not gospel, of course, but I'm more inclined to believe larger moves matter more than smaller ones.
A final observation: depending on what type of bullet you are using, .003" of seating depth variation may be too fine for you to resolve as the bullet may not be consistent enough, even measuring CBTO instead of COAL, to seat to the same measurement every time. Nosler Accubonds and Berger Hybrids are not in the same class when it comes to consitency, for example, though Accubonds usually shoot plenty accurate enough for my hunting uses.
Again, I don't care what you do. If you want to do a bunch of seating depth testing, by all means. I am personally guilty of testing different loads and different things just to see if it makes a difference I can see, or just to try someting new, even when I've been told it won't matter (or that it will). And the Lord knows I've spent way too much time and money trying to get the last bit of accuracy out of a load or barrel. I'm just trying to save you some trouble if you don't want it---I don't think very many people actually benefit from seating depth tuning. The bullet either shoots in that barrel with that powder, or it doesn't.