I'll say up front, I'm no expert on Discharge and % of activator. Having said that, I don't think that is your best approach. Most people approach it from the art side. There are several ways to achieve a aged or distressed look just using mesh selection.
A few examples:
1, based out ink, on a tri blend or textured shirt, on high mesh with light colored ink to let a lot of the shirt show through the print.
2, Google the words Free distressed textures. Also Aged or vintage distressed texture patterns and use those to knock out of the art (straight to shirt color). You can also randomize this pattern distress by knocking out of the colors...and only 50% of the base...and then use another texture to knockout 100% from under the base. This gives the print/colors some variations you might see in some more authentic vintage looks. If you need guidance on how you knock those out of the seps is another post.