In the summer I bought a Rokinon 16mm F2 lens for my canon 400D alas the canon died and I'm now investigating whether I can connect it up with my QHY8L instead. I can't find any details about spacing but I knew roughly and got this result from a 25sec exposure (alas the lens had misted over).
(https://farm5.staticflickr.com/4502/23897792558_48346d47ea_b.jpg)
As you can see there are black shaded areas on either side of the image (not the border) is this caused by the lens being too close or too far away from the sensor? My natural thought is too far but it's so long ago since I've used my kit I have no confidence.
if you look at your camera you will have the focal plane marked on it, normally a circle with a line hthrough it, this is where your (film) sensor is.
The distance from that point to the front of your lens can be measured with a ruler.
If you then fit it to your QHY and make sure the distance from the sensor to the front of then lens is the same you can use this as a starting point,
You might find that you will have to move it a few mm either side.
One other thing, do it in daylight, much easier to set up and adjust.
Mac.