What would you use to sand, reduce the thickness of a flat rubber washer? Sandpaper? Most likely I only need to remove 1/64th" at most 1/32". I have plenty of washer so trial and error will get the right thickness sooner rather than later.
Experience with this group dictates that I must explain the why. :)
We have a transfer valve on the garden tub. The shower head broke. The part is obsolete, replacements from Delta are made differently and because of the deck mount would not be a good choice. Changing out the deck mount would be a major project. I bought a hand held shower unit at Lowes, but the thread run length is slightly different and the old washer was bad. The OEM washer is also obsolete and did not even have a part number. I substituted a regular flat washer but it is slightly too thick resulting in the angle of the dangle being wrong.