It’s slightly understated. The 3lb is stated at 0.15 but last time I checked admittedly a while back it was more like 0.16. But it breaks at well over 4lb so it’s actually pretty low diameter for it’s BS.
I fish for bites and make sure my tackle is balanced ie if using light lines I will use a light actioned rod that will act as the shock absorber. No point fishing 10lb line just because there are 10lb Barbel in the river, chances are you would probably sit all day with no bites. It is a case of being realistic with the expectations of what you are going to catch, I fish a stillwater that holds a few 30lb carp but I would normally expect to catch fish in the 4oz to 5lb range, so I fish accordingly as the chances of me hooking one of the large carp are pretty slim so I fish with tackle that will get me more fish on the bank and deal with the occasional bonus fish when/if it comes along. For me it is all about balanced tackle get that right and you give yourself a good chance of landing the occasional bonus fish. Using light line on a powerful rod will eventually end in line breakages and also using heavy line on a light actioned rod risks rod breakages
Agree but you must still have some idea the BS so that you are not out of the ball park matching line to intended size of fish. Diameter tells you what line is thicker/stronger, thinner/weaker but you still need BS to start with.
You cannot start, say with 0.06 line, ignoring BS and keep increasing thickness until you stop losing 5lb carp. You must have started with a BS that seemed suitable and then adjusted as experience dictates.
If you've worked with a scale (either BS or Dia) for long enough you will 'know' what is what and match mainline and hooklength. I never worry at all about actual or quoted BS as I 'know' what is reasonable for my use. If my mainline is 0.16mm (waggler let's say) I'm not ever going to be using a hooklength thicker than that (for my'normal' use it's unlikely to go above 0.14mm and that's chub fishing. Mostly 0.14mm mainline to 0.08mm - 0.10mm hooklengths. Don't know what the BS are exactly).