This doesn't really make sense to me, when I do event work unless it's a dungeon if yiu use say 1/150s, iso4oo and f5.6 the flash is more than powerful enough to light the subject and these settings usually register ambient also

maybe try matrix metering and put in manual, iso400, s/s 1/150 and then use the apperture to control the ambient?

Or even p mode just works, or should