Those GN are basically the same. The MAX reach of the flash is the GN / aperture f-stop. Let's make the math easy, we'll use GN 44m with an aperture of f/2.0 (you have a little further going to f/1.8).
The GN is not regulated. It can be at various ISO (It is usually a rating at ISO 100).
It can be different at different lens focal lengths. Some flashes are rated at 50mm focal length. Others use the rating of their max zoom - let's say 105mm.
For instance a flash may have a GN of 44m at 70mm.
It might perform at:
GN 52m with 105mm focal length (if it zooms that far)
GN 44m with 75mm lens
GN 37m with 50mm lens
GN 32m with 35mm lens
So let's assume you have a flash with GN44 that covers a 75mm equiv lens (50mm on APS-C) to:
44m at f/1
22m at f/2
11m at f/4
But what about the range when power is not at 100%?
One quarter the power is half the distance. For instance:
GN 44m reaches to 22m at f/2 with full power.
1/2 power = GN 32 and reach is 16m
1/4 power = GN 22 and reach is 11m
1/8 power = GN 16 and reach is 8m
1/16 power = GN 11 and reach is 5.6m
1/32 power = GN 7.8 and reach is 4m
1/64 power = GN 5.6m and reach is 2.8m
1/128 power = GN 4 and reach is 2m
1/256 power = GN 2.8 and reach is 1.4m
Many flashes can use even lower power in automatic TTL mode than their lowest manual power setting. But you seem to want full manual control for consistent results.
So you can see you need lower power levels if you're using close subject distances.
Lastly, to not have an overexposed subject lit by flash and a dark background not lit as much you would use slow sync.
Basically, you would calculate the shutter duration required to properly expose the background with no flash. If the camera is set to this or 1/3 stop less your flash will just kill shadows. If you set the camera to 3 stops less than the ambient light, then your exposure will be effectively just light from the flash.
Balancing the proportion of ambient light to flash light in the exposure is basically done by adjusting the iso/aperture/shutter speed from -3 stops to the -1/3 stop for the ambient light. At -3 stops exposure ambient is dark and you only light up however strong the flash is set to.
Using our GN 44m example, at 1/16 power and f/2:
Subjects much closer than 5.5m will be overexposed
Subjects near 5.5m will be properly exposed by the flash
Subjects much beyond 5.5m won't be lit enough by the flash
Subjects (background) beyond 10m will be mostly lit only by your ambient settings controlled by exposure time ("shutter speed").
Lastly, flash sync (aside from HSS - fast overlapping strobing - or a leaf shutter).
Your APS-C camera will have a flash sync speed. Let's say 1/160 second. Faster shutter speed than this won't work. Slower shutter time than this brings in that ambient light. Meaning, you can't use your 1/4000 sec highest shutter speed with regular flash. Nice thought, but let's just say you can't go faster than your flash sync speed.
Like you understand, the flash duration can be thought of as short but the way the fast shutter speeds are achieved is that one curtain closely follows the other one like a gap or slit. For instance, at a 1/800 shutter speed setting the first curtain starts to open. Once it gets halfway open, the second curtain begins to close behind it and is now "chasing" the first curtain across the sensor. When the 1st curtain gets to fully open, the 2nd curtain is already at half closed. If at this moment the flash fires (let's say for your flash 1/128 power is 1/20,000 sec) you have half a picture that got flash, other half got none. The 2nd curtain continues and when it's closed the exposure is over.
At 1/2000 sec, the 2nd curtain started closing when the first was 10% open. Thus 1st curtain is 50% open and 2nd curtain is 40% closed. This 10% exposed gap moves down until 1st curtain is fully open and 2nd curtain is 90% closed. Flash fires. Only 10% of picture saw flash.
At max flash sync speed of 1/160 (or whatever your camera has, this depends on curtain speed), you have first curtain opens fully - it is open then flash fires
- then 2nd curtain starts closing (1/160 second after 1st curtain started opening).
In one millisecond (1/1000 sec) increments:
0ms - all closed
1ms - 1st curtain is 1/3 open
2ms - 1st curtain is 2/3 open
3ms - 1st curtain is fully open
3.01ms - flash fires
4ms - flash is either finished or finishing at a much reduced brightness
5ms - flash is either finished or finishing as shutter remains open
6ms - 2nd curtain starts closing
7ms - 2nd curtain is 1/3 closed
8ms - 2nd curtain is 2/3 closed
9ms - 2nd curtain closed exposure finished
The 1/160 sec (6.25ms) exposure took almost 1/100th sec (10ms) to fully occur. The 2nd curtain followed the first curtain by a delay of 1/160th sec (6.25ms).