The real world is not so perfect. I never see Heisei or Showa, or earlier reigns as a single character. The Japanese are also not fully onboard with Unicode so there's an extra level of effort needed if you want to get into a normalized form.
For this specific question, I strongly suspect that whatever the length of your Heisei string is, it's safe to assume that the new era string will be the same length.
The code point was already reserved in unicode 11.0, so you could even just the unicode code point.