So from what I read about King betta genetics...
Kings are not giants they are just larger splendens?
The only Kings i have seen are HMPK's.
When breeding a king and any splenden, you get 50% kings and 50% regular. If somebody was to continuously breed a halfmoon female with a King hmpk would(after many generations of inbreeding and finding other female Halfmoons eventually elongate the tail of the king?(if all none kings were culled/sold)
So the pairs would be HMPK King * HM female. Then best tail king * hm female. And continuously get the best tail and a new female(preferably a single color to keep the colors nice).
Does any of that make sense?