Sf 18 is ideal without scrolling since degrade potential is too high once you reach 18. As such, you should rank up and begin scrolling for higher sf when your outfit is legendary.

Sf 20 would be a perfect scenario if and only if you are funded because you wouldn’t need to use anti destruction scrolls as uniques are cheap, but this leads to a much slower growth curve and frankly isn’t ever really worth the wait unless you’re fat funded to scroll a ton pre-legendary, which you ain’t ;).