There is no universally accepted method..

No matter the brand or the size marked you will be able to disprove that "Measurement" with the next razor..

Some Makers measure one way and another can measure a different way, to be totally honest and accurate you have to measure Top to Bottom otherwise there can be a judgment call involved about where the "Hone line really is (Think about a wider spine wear line)

But honestly just so long as explain the measurement then it can't ever be really wrong..