The shading model is, as far as I know, only responsible for calculating how the material influences any light that hits it; it doesn’t have to do with transparency.
Unfortunately, the masked mode can only make parts visible or invisible and ignores everything inbetween. If you need something visible for only 50% (or any other value but 100% or 0%), you are pretty much limited to the translucent material mode (and additive, if I’m not mistaken, but I reckon you are not looking for a glowing dress). I remember reading somewhere the reason for the limited functionality of the masked mode had to do with the deferred renderer they are using in UE4.