AWH (and JT); I'll reply tomorrow in detail.
For now, let me share my comment to my airport pavement engineering students - "no-one can easily and intellectually understand ACN and PCN, so I suggest you work through the tedious and turgid process and then keep a worked example to copy each time you need it".
If you ever get comfortable with it, then please let us all know how it works.
Having said that, I have derived a couple of ACNs from first principles lately - for the Shorts Belfast (the manufacturer's chart was wrong), An124 (COMFAA was [unusually] in error) and for the ATR42.
I reckon that COMFAA is the greatest asset to any pavement engineer ever, and ACN and PCN have become much more accessible as a result.