Thanks Alex, I've been wanting to speak on this a bit.
To give a bit of background, which may be relevant, my performance on rock or a board has always correlated very strongly with finger strength scores. In my head I put this down to a few things: 1. finger strength is/was my weak link, 2. my body strength and mobility is very good so I can readily apply any finger strength gains to the wall, 3. I think i'm quite good at picking up on the nuances of a hold and how to get the most out of it and also rinsing body positions to put as little weight on my hands as possible. That is to say, yes, the active pulls correlate well with my climbing performance, but hangs did too.
I'm not surprised at all that your day-to-day variability on active pulls (lets go with this as the name) is high. This is because they provide a better measure of fatigue than hangs. Active pulls predominantly rely on recruitment at the finger flexors, whereas hangs benefit from passive tension within the muscle, tendons and other structures. This fact means that you can usually achieve similar hang scores even while fatigued - I see this as a negative because it's digging a deeper recovery hole whilst loading the passive structures more heavily and not reaching optimal recruitment at the muscle. This is readily accepted, because other sports are known to use a grip strength test (actively squeezing a device) as a measure of readiness to train.
Making this specific to your training, I understand you do a lot of endurance work (from PE to ARC), so it's likely you carry fatigue from one session to the next? Personally, my active pull scores are fairly consistent because I do basically no endurance training at all and my weekly schedule is consistent. However, when I've had a big day out on a weekend, my pulls on a Tuesday are still affected sometimes. However, I don't understand why you are still able to get high scores when you're in worse form, unless you were well rested at the time of testing.
On form, I don't fully isolate my fingers with equipment, but I try to just pull with the fingers. I did try a setup with my elbow on a table but didn't get on with it. For the benefit of others, what I do is attach a large full pad edge (26mm) to a Tindeq anchored to the floor, so that I can hold the edge at a comfortable height with my legs and elbows locked out. I then pull to peak force just by contracting my fingers and not by leaning or pulling with my body. I say peak force, but what I actually do is pull to 95% of my measured peak force and do this for around 5 reps (at this point i'll usually have dropped under 95%). That would be one set and is what I did for the first few months seeing great progress, a corresponding increase in performance on the wall, and interestingly my time needed to warm up for every climbing session drastically reduced, whether i'm doing the active pulls or not on that day. I've always been very slow to warm up, but now i'm at almost Will Bosi levels of readiness, able to pull hard almost straight away!
For the last few months, I've introduced concepts from French contrast training, which I first read about from a rugby coach. Before performing (in our case this would be climbing on rock or even a board session), the idea is to do a strength exercise followed by a power exercise for a few sets, a few hours before performing. What I do on this session is pull to a subjective 90% of peak force for 3 reps without looking at the Tindeq reading until afterwards, then after 1 minute rest, I do 5 speed pulls using the same edge and the RFD program on the Tindeq, trying to get the best scores I can. This is 1 set. On set 2, magic happens, the two exercises potentiate each other, my 90% is now higher and the speed pulls are higher. I get stronger and faster each set, and when I'm just starting to plateau I end the session. The first time I did this I hit a new PB whilst subjectively thinking I was at 90% of peak force! A few hours later I had an amazing session on rock.
As of last week, I added some hangs in again for the first time this year, more for curiosity than anything. I'm doing assisted 5 second 1 arm hangs on the BM2K middle edge and i'm really pleased to see that I'm a lot better at it than I ever was.