Oh I had missed that your rim was also in a wheel currently in use.
You can do it without fully disassembling the wheel - though you do have to take the tire and rim tape off so it's still a bit of a faff.
This video from the 5:20 mark does a good job of showing the method. He calculates the outside diameter by measuring the radius - usually you can get away with just adding 10 mm to the ERTRO bead seat diameter.