Ok, so that worked when I used my Purchase dimension so I did get a static number for the date range - which is a lot better than nothing. Unfortunately it doesn't let me trend that value over time in a time series. Not sure if that's possible.
Yes, but when I create that and add it to a table it seems to simply provide a SUM of the date range selected for revenue and not a median at all. These 2 values are the same: