I will modify my adapterplate since the starter motor gear is too far from flywheel gear.
What distance between starter motor gear and flywheel gear at idle would you say is suitable when the started is idle?
2-3-4 mm ?
You need to be setting the starter motor on the engaged position into the ring gear, not the idle position. This way the correct engaged position, which is the important bit, will be correct
Bench test and run your starter to watch bendix travel and make note / measure. Hang your spacer plate on the back of your block and mount your starter on the spacer plate and mount your flywheel. you should be able to determine if you have a proper fit.
If you're talking about what amount of clearance is appropriate between the starter gear and the ring gear, you don't need much. Just a few thousand should do it, measured from the face of the engagement.
Now I have measured and tested the starter motor on the bench. With the following approach - take off 5 mm of the adpater plate for starter motor mount.
This should give 3.2 mm clearance to the ring gear and 10 mm grip on the ring gear when engaged.