@solikaldorei
I'm assuming you know that transparency is achieved with varying shades of gray. The only way I've found to be able to get just the right amount of transparency you want is by trial and error. I usually start with this file which trial and error has shown me to be about 50% transparency:
This is my test image with complete transparency (black) and 50% (gray)
and work from there. Changing the color of your OPAC to either a darker or lighter gray will increase or decrease transparency. Black is invisible or completely transparent, white is opaque or no transparency. Once you upload that texture into the OPAC slot next to the texture, apply your changes and the first texture will go transparent. By applying the gray shades to different areas of your OPAC you can make some areas transparent and others not or less see thru like stockings which are often less transparent at the top of the thigh than they are down the rest of the leg and the toe and heel area can then be made less transparent again.
Checking the blending box can make the effect smoother, but it can also cause clothing items to disappear when viewed thru other transparent items like windows or water in rooms so only check blending on if you really need it.
Hope this helps.