Try photoshop. Play around with Noise and Dust&Scratches filters, then use the image as a transmap. What makes a realistic translucency isn't the numeric transparency setting so much as the composition of the material itself. Paper has bits of thread materials, miniscule grains of wood pulp etc. The wrinkles in an animal's skin, minor scarring, and simple genetic pigment variations will make parchment a much varied transparency. Unless you want to duplicate celuloid or some other synthetic, you will need an image transmap or a good python script to get a quality render of a translucent material.