Except that Japanese eras actually go back to approximately 645 AD, and some emperors reigned through multiple eras and others shared eras (Emperor Meiji himsel

Except that Japanese eras actually go back to approximately 645 AD, and some emperors reigned through multiple eras and others shared eras (Emperor Meiji himself began his rule in Keio year 3, and it wasn't until Keio year 4 that that era ended and the Meiji era began). Your scheme only works for modern eras rather than the full span of them.