Hmmmmm. Try 52mm, or if you have the adapter, put it on the spindle, and move the tonearm so it's centered on the spindle. Line up the stylus with the mark on the adapter, and tighten slightly to hold the mark. Lay it on it's side and measure from the rubber washer to the stylus tip. It might be easier to lay it on it's side on a piece of paper and make 2 marks on the paper and then measure it.
Write it down somewhere you know it'll be safe (like the inside of the owners manual). Now you can use that measurement to set it any time without resorting to breaking out the adapter and setting up the process.