Night time may use less since the sun won't be doing quite as much to the FC but it is hard to predict how much less. Just a guess here but it looks like you lost 3 ppm in 3 hours? So if you are going to leave it untested for 10 hours, and using the current FC consumption rate, 10 hours X 1 ppm/hr + 21 ppm + 2 ppm (for safety margin/higher kill rates at higher FC) = 33 ppm. A general rule of thumb is if you are below you mustard algae shock level (I get 44 ppm FC for you 80 ppm CYA), you shouldn't harm anything. Leaving it that high (44) or higher for long periods of time could cause problems.
Once you get a night under your belt, you may be more capable of predicting what to put in the next night.