I know exactly what you mean Pete about that grey area during and after fundies where you know what 'not' to do deco-wise, but don't have the knowledge to do it differently.
I hated using a dive computer from my first dive with one - long before I even heard of DIR. More than anything, I hated the fact that I didn't understand it.
I've used the analogy several times since that I would hate to have to use a calculator without any knowledge of maths. If you understand the basics you can spot if somethings not quite right....
345 x12 = 690
?
Nope - missed the first digit off the multiplier - whoops - do it again....
See what I mean - it's OK using computers/calculators but not if we switch our brains off when we do so. We need sufficient knowledge to understand what they are doing - to check that all is well.
I had two instructors on my fundies course (AK and RichW) and got quite cross with both of them when it emerged that the course does not cover anything beyond air tables and minimum deco (for the purposes of gas calcs only).
Being told to 'dive the tables you learned to dive with' is not much help when you (like me) learned from PADI that 18 metres a minute is an acceptable ascent rate and stops are not 'required' until approaching no deco limits.
But they were right to say this. DIRF is not about teaching everything in one go - nor can it be.
So when?
Play with deco (in your head - not in the water) look at and learn the no stop times and see what patterns emerge. Shove figures into Decoplanner - and see the same (don't forget we're still talking minimum deco recreational diving here). As you do this more, you'll get more confident about where the 'arbitrary' limits are - I say 'arbitrary' because that's all thry are - we have no idea what suits us individually - but then your computer has no idea what suits you either.
For me, my computer got put into gauge mode permanently on Tech 1 - but only after I bent it twice
Fraser and I headed off to the Red Sea a few days later and did a load of those multi level repetative reefy type dives that everyone does there (plus a couple of deeper ones).
Could we have used computers - sure - of course we could. In the same way that I COULD take a calculator to the shop to work out my change when I buy a newspaper - but as both are equally simple - it's nice not having to.