You'd have to know the guide number of the flash, plus have a light meter to get a reading of the scene. Using the reference chart that's usually attached to the flash, set it to your film's ISO speed, read the distance to the subject using the camera lens' distance scale, then reference the required f-stop needed on the flash's chart. This would be an aperture required to fully illuminate the scene at the focused distance; but what you want to do is to under-expose the flash exposure, mixing it with an under-exposed ambient exposure.
So, set your camera's aperture and shutter speed to 1 stop under-expose the ambient scene, at an f-stop that 1-stop under-exposes the flash reading. This will give you a 50-50 mix of ambient and flash, at the focused distance.
This is all assuming your camera has a leaf shutter and syncs flash at all speeds. With a curtain shutter, the calculation gets a bit more complicated. 😉
Example:
Flash recommends f/11, ambient meter reads f/11 at 1/250 second. Set the camera's exposure to f/16 at 1/250. This will under-expose both the flash and the ambient by 1 stop; when both exposures are mixed together, you should get nearly a normal exposure with a mix of flash and ambient lighting.
~Joe
EDIT: I'm not familiar with the 7ii's metering system - does it give you an indication of shutter speed at each aperture setting, before taking the shot? If so, you can use it as your default ambient light meter.
