I never liked the way hairs created with FFX look like - at best, like a wire, very thick, etc. I used a transparency trick (gradient on the parameter Fiber V) which softened the look of the hair. Nevertheless, the hair remained rather coarse. Since the smallest thickness that can be assigned to hair is 0.0001. Which is equivalent to 100 um.
But the other day I accidentally entered the value 0.00005, and although this value immediately began to be displayed as 0.0001, the effect was made precisely from the value 0.00005. Accordingly, there is only a limitation of the LW interface, but the values may be less than the LW interface can display. As a result, I can finally get more interesting hair with FFX.
Good luck!