I know the technical aspects of controlling flash power: increase or decrease flash exposure compensation, and if that doesn't work, increase or decrease shutter speed, and use more flash power for longer focal lengths. I understand this.
I wanted to check if there's any guideline to find roughly the right range of flash powers for a given scene, rather than painstakingly trying out a couple of dozen shutter speeds from 0.8s to 30s to find the one I like. Is there a rough analog of the Sunny 16 rule, but for flash power?
For example, I tried taking a photo of my garden without flash:
And with flash, which overpowered the scene, resulting in a bad photo:
By trying various shutter speeds from 0.8s to 30s, I found that 8s looks best, filling the dark foreground without making it obvious that a flash was used:
Are there any guidelines to make this process less painstaking than taking a dozen or two photos each time?
Note that I don't have off-camera flash, or multiple flashes, or umbrellas, diffusers or other accessories. Neither do I have the ability to change the angle of the flash on my camera (Sony NEX-5R). The only variable I have control over is the ratio of flash to ambient light.