It takes time for the hormones to build up to induce flowering. It requires the full 10 hours or more, and increasing darkness daily, minimum with most strains, to get them to even begin to show signs of flowering. And of course we generally use 12 hours or more of dark, to keep your plant healthy in flower, as often under a stable indoor light schedule, even 13 hours of light and 11 hours of dark can often keep a plant in veg indefinitely.
And so it isn’t the ratio of light, as much as the duration of darkness during a 24 hour period.
There is a technique called the Gas Lantern Routine, that can guarantee a short day plant will stay in veg, even with as much as 11 hours of darkness during a 24 hour period. And it is quite common in general horticulture teachings, most college level horticultural books will mention it, for short day plants, to be able to keep a plant in veg. And the technique is to interrupt the middle of the dark period with an hour of light.
12 hours of light, then a 5.5 hour dark period, then a hour of light, and then 5.5 hours of dark again before the light comes back on for the regular 12 hour day period.
This technique is often used in greenhouses to keep a plant in veg, even when the outdoors conditions do not give it enough light to stay in veg.
And as you can see, if you kept your plants on either the 18 minutes on and 6 off, or 12 minutes on 12 minutes off, the plant will likely maintain a veg light cycle hormone level and therefor never flower under such a schedule.