Since the flash has a GN of 25 @ ISO 400, there is no need to correct for the different ISO. You want to use ambient light for your base exposure and the flash for fill. You can estimate the balance of ambient to fill as a ratio: say 3:1 you would not want the flash to be more powerful so you can reduce the amount of light from the flash. The flash burst from electronic flash is quite short and Theoretically you will capture all of the flash's light on film if the shutter syncs properly.