Getting the fly to the desired depth can always involve some trial and error. With experience, I can usually look at the water I'm about to fish and estimate what sink tip I want to use and whether I will use an unweighted or weighted fly. I'm finding my 15' type VI to be nearly universal, unless the water is significantly slower than average, in which case I switch to a type III. And if I can get deep enough by angling my cast straight out or slightly upstream, then I use a weighted fly. If that doesn't do it, I'm probably not fishing good fly water.

Sg