Therin lies the problem Baz as long as you work in lbs BS them different nylons/fluorocarbon/braid/gut.....whatever will not give a true reading unless we talk about the same make of tippet. Dia is much more sensible IMHO though you then get into stiff or limp or semi stiff....etc.
Solution?Ttry a few and stick to the ones you like
Yup rod action and rating also critical. Fast tip action #8 might not suit a light leader material.
Nylon diameter more relevant than BS as Peter rightly states, but also take into account 'glare' and stiffness as these can have a huge bearing on your fishing.
FWTW i fish 3lb Maxima green on rivers (and lochs now as well most of the time) for all high water / surface work, with 4lb Maxima green for wild loch wets and lures, and rainbow dries.
6lb Riverge for sub surface rainbow pulling stuff.
#5 mid action Streamflex Plus 9' 5" extending to 10' rod on rivers and wild lochs or 10' #4 Streamflex (often using the extension piece out the Plus) on float tube, or for specialist river stuff (French Nymph, bugs etc) The action and rating of the rod allows me to fish much finer.
#7 10' 6" for big water stuff which means i need to fish higher rated tippet, again though paying attention to diameter etc so as not to impact on fishing as much as possible.
Slightly confusing i know but hope this helps a wee bit.
Allan