I wouldn’t worry too much about trying to measure it. The inner thrust washers only come in 3 sizes so whatever you measure the end float to be, in the end you have to compromise with one of the 3 washers.
If it was me I would buy one of each size (they are not expensive) and just try them all starting with the thickest. Apply finger pressure to the out washer and if the clutch centre gear spins freely and/or has detectable end float then you probably need to replace the outer thrust washer which only comes in one size. Otherwise repeat test with the next size down etc etc. What you want to achieve is a gear that spins freely but has as little end float as possible. You will find your fingers are very sensitive to movement and binding.
One thing to be aware of is that these thrust washers do wear on the inner cutaway section so measuring the overall thickness is not always a good indication of their operational thickness. Sometimes all you need is a new one of the same thickness as already fitted. Same applies to the outer washer.