Do you mean that f2 of 15/9 flowering male and female will flower at 15/9, 16/8, 17/7 and 18/6? If so, do you know what percentage will flower at 18/6? Is it 25%?You can expect intermediate results in f1 hybrids, i.e plants would generally start flowering at a 15/9 light regime which is late. This kind of hybrids work in Denmark to some extent but assuming you're north from there you better start with something earlier. Make sure the indoor mum flowers at 14-16 hours and the outdoor dad at 20-22 hours and you'll do better.
F2 will give you the whole range from early to late