I usually give them a 24 hr dark cycle if i have space, then harvest when the lights come on, but i have harvested without the 24 and had great buds so i dont know for sure what effect it has.
Sorry wish i had a better answer but i cant confirm something im not sure about. I heard some people go 48 and 72hrs but i never can wait that long lol.
Try it with clones that way you have 2 samples of the exact genetic profile and flowering time and can make a good comparison, you can even send them to a lab and test them for cannabinoids.
Peace out and good luck