What wattage is your HPS?
How far above the canopy is it?
Average temps and humidity?
You're doing fine by the way. I just prefer 18'6 or something close to it, over 24/0.
I think it depends on your light intensity and how much light your plants want at the time. If my veg lights are on 18/6 and I notice my plants going to sleep (droopy leaves) an hour before lights out every day, that tells me I can go to 17/7. I just reset the timer to 16/8 yesterday.
I like to listen to my gut a lot which leads me to do a lot of things some growers frown upon. I have a big gut so it has a lot to say.