Depends on your overwad. If you have a strong, hard, stiff wad then I just apply enough to make a meniscus around the brass touch wad area - like a o-ring would fit.
If the wad is less substantial, then a 1.5-2 mm layer.
I just cautioned in regard to the layer glass must be short enough to be able to flip over in the shell, not thick enough that it became a plug.
Like a coin can flip in a precise fitting pipe, but 3 coins glued together can not. I do not know if it is important - but it worried me enough.