Ok, so you are going to get a mix of ambient and flash. The ambient light should be a relatively steady source. The flash bulb will be a known amount of light. You have two options to change the balance of ambient to flash. First you can use some sort of filter over the flash lamp and reflector. Many years ago the photographer may have used something like a linen handkerchief to reduce and soften the flash. The second way to reduce the flash output was to move the flash bulb farther away from the subject. There are auxiliary flash units for this purpose. You need to run a cord from the synchronizer to the "side" flash unit.