Can you change the integration time to a 2 digit decimal? It would be more accurate, especially for 15min intervals.
ZWO LUM
: 3x30" (gain: 139.00) -15C       = 90sec
ZWO S-II: 9x90" (gain: 139.00) -15C         = 810sec
Integration: 0.2 hours                              = total 900sec  = 15min

https://www.astrobin.com/327958/?nc=user