How to change PWM_MAIN_MAX5

Hi friend,
my firmmware is PX4 1.9.2 and i need to change PWM_MAIN_MAX5 to 1200 us but system repond is impossible and minimum valor is 1600 us.
I need to change for trigger camera Sony whit SeaGull #REC.
I think my problem is difference whit trigger valor of SeaGull #MAP2 1700us (range 1600-1800us) and trigger valor of my SeaGull #REC 1200us (1132-1265us).
When i trig from QGroundControl trig video mode… in my SeaGull #REC just valor trig of 1800us (1601-2200us) for video mode.
Very thank’s to all
@LorenzMeier