I have always found the punnet square to be confusing for bettas - breeding results doesn't concur the theory. Perhaps this is due to excessive cross breeding thus all have mixtures of genetic codes. Never the less the punnet square or the color genetic theory is useful as a basis of probabilities.
I don't know any breeder, local or otherwise, who has produced 100% giants. The most giants produced is about 30%. 70% is made up of normal regulars and those that suddenly stop growing (usually they have giant form - wide body).
BTW, the term "1/2 giant" is only something I use for those that can't grow to max giant size but carry giant genes. Locally they're all called giants. Though they can produce true giants but most of their offspring will be half giants.