Hi Gerhard,
I had a similar problem over a piece of work I was trying (making threaded studs for a horizontal Sterling engine), but I do not have a spindle depth stop for the lathe.
Obviously, if I had one - like Bogs is making, its an easy job to do.
This is the way I overcame the problem of getting a repeatable length - it may not be a recognised way - but it did work for me and is extremely quick and easy to do as a one-off set up.
I used the tailstock and its drill chuck as a sort of "adjustable stop".
Put the bar material in the lathe chuck.
Put the drill chuck in the tailstock with jaws closed and lock its spindle, held a small scrap of steel plate against the closed jaws of the drill chuck, pulled out the bar material from the lathe chuck to touch against the face of the scrap plate (being held hard against the closed jaws of the drill chuck) to the exact length I wanted,closed the lathe chuck jaws and locked both the tailstock and the saddle (with parting off tool at the other end close to the lathe chuck) - the exact point I needed to part off. Take the bit of scrap plate away and part. (The bit of scrap plate - spacer - is used to stop the rotating end of the material being intefered with by the jaws of the tailstock drill chuck when parting off). repeat for the number of lengths you want.
Obviously you cannot use this method if a drilling operation is also required on the length of material, but it seemed to work for repeating a "solid length".
When threading, I used my set of drill bits - bought a set of metric drills at 0.1 mm increments - as "slip gauges" to set the saddle stop from the edge of the saddle to a precise length, and locked the saddle stop - easy repeatability for cutting the same length of thread each time.
Forgive the garbled text - but I hope you understand the idea.
Regards,
Peter
