Like I said, my E30, CTR, Singer all have pretty well made STA rear suspensions. You guys are free to DL them, open them and look at it or tweak it. I can give you the curves for them if you want and maybe some help if you catch me at a good time.
If there don't exist curves and good pictures for the suspension, it can be a little difficult to make it very good, but luckily most STA (That I have seen) are very similar in design.
With just a few pictures you should be able to guess it pretty well, if you use an existing geometry as a guide and know about what it should come out as. If you know the sweep angle (Angle between the inner and outer chassis point) then you can also calculate the camber/toe angle vs trailing arm angle, so even if you don't have curves, you can do a lot with just math.
The motion ratio for it isn't calculated easily like in normal DWB suspensions, you will probably need at least a picture, and it would be possible to guess pretty well from that if it's a good one, but the best would be a clear picture from above, like a drawing. You can then get a pretty good MR estimate with some geometry.
Of course, the best would be a roll center, curves and MR from the manufacturer or from a good measurement, but I don't know if it exists for this car. For my car, I had all of those + good pictures, so it can be more difficult for you, maybe. Even an approximation is not so bad though.
Also I don't suggest to use the KS ones as a base: the E30 and CTR are just wrong and the RSR also is different from a real 930 STA.