That is a very cool idea! You would need a cap tube ID gauge of course. But then just unbraze the ends, get the length, and you can work out the capacity of common refrigerants. Somewhere I have an extensive chart showing all possible combinations but Supco probably has a conversion chart from non-Supco cap tubes to Supco cap tubes. I know some people dislike Supco but I have had good luck with their cap tubes and sizing.