Very intersting piece of advice. After your previous message I dialed back my last dive session to what felt absolutely no stress so 15m max. But I did do seriously more dives than you suggest so I will keep that in check as well.
As for the Frc for less deep divers, it's not so practical because I don't want to be annoyance to boats or even risk being hit so will consider. Why are you so strict on the # of dives? Is it bad idea to do eg 6 10 13 15 10 10 10 10
I spoke in depth with Aharon Solomons about this.. He has plenty of experience with RV training and came to similar conclusion that more than 5 is a lot, and then having a similar volume-decay to what I said.. depending on depth achieved.
The reasoning is that, any dive (literally) below your RV causes some 'micro-damages' to the lungs (comets on ultrasound), and strong bloodshift (may) cause free-radical damage (this is debatable but there strong logic to the argument).
Point is. If you over-stress the body you will not 'adapt' because you spend all your adaptive energy on recovering BACK to baseline, instead of recovering and having supercompensation ABOVE baseline.
Aharon's opinion (where I differ) is that FRC is completely useless & that you should only do RV after achieving 50-60m. His reasoning is that until you can do 50-60.. even 3m on RV is so much more pulmonary stress than you've ever experienced, and that this can cause the above issues i mentioned with adaptation.
My opinion (building on Aharon's, which is how things develop) is that you can bridge the gap to RV, by doing FRC dives to about 1/2 your PB or goal, until you reach about 50.. (so FRC up to 25m before transitioning).. This will give an 'entry' into exhale diving (so your ready by the time you get to 50 and can use RV) and is an effective way to train Frenzel and beginner-mouthfill when it's applicable to this level (let's say 30-50m)..
Again for adaptive issues (why I have my intensity -decays: 20m no warm up RV for +80, 25% of your goal on RV (50-80m) is because any exhale (RV & FRC) is more 'aggressive' (purely in term of compression,) than full lungs diving.. This is why it works, but also why you shouldn't go 'significantly' past your goal lung volume..
Example: Goal to dive to 60m = 1/7th TLC
20-25% = 12-15m RV = about 1/8-9th TLC (which is already more than you 'need'.. but this surplus can be useful once you start adding in other factors like CO2..)
But if you were to do 20m RV= about 1/12th TLC.. this is a bit too much stress in my opinion.