For what it's worth, I experimented quite a bit with knots and F'carbon. To cut a long story short, experiment / test with many knots, and you will also probably find that the thicker F'carbon (and probably the most expensive) knots best!! no surprise there

Fine F'carbon is very delicate when knoted and some "pros" say use 10lb most, if not all, of the time. If you get good at your choosen knots, consider using leader rings if you go to finer stuff...Riverge (maybe the best F'carbon?)recommend them in the literature in their packaging.
Finally, I tried Incognito F'carbon (used mainly by Carp men I believe?) and would say it's probably the best value (not cheap either!) all things considered...just my opinion...you will find every angler has their own favourites for one reason or another. Hope this helps