Shot myself in the foot. That's what I get for trying to quote from memory when in a hurry. Dug out my stuff, here's the correct data:
Original DWM recoil spring for the 7.65 mm
19 turns
Free length: Nominal 85.00 mm, Min 82.50 mm, Max 87.50 mm.
Pre-load at 50.00 mm, min 15.43 lbf, Max 19.84 lbf
Loaded at 34.00 mm, min 33.07 lbf, Max 36.38 lbf
Note that in the previous post I misquoted the minimum safe preload. When cutting a spring, the preload drops faster than the loaded force. Since the pre-load force has a large influence on the final closing you can get into trouble. The Wolff spring has a free length much longer than the original so you can get away with cutting it.
The spring must completely close the action every time without fail regardless of load. This is a safety issue as the Luger can be fired with the action not quite locked. This makes the minimum safe loaded force 27 lbf in a standard length uncut spring for either caliber, no matter how light the a squib load.
Reliable functioning will depend on having cartridge length match the magazine as well as the impulse matching the recoil spring. The Winchester load is a tad lighter than the original, the Fiocchi lighter still.
Don't forget the main spring. (The one inside the breech block that drives the firing pin.) This one has an influence on final closing also. The loaded force should be 4.5 to 7 lbf.
testing the recoil spring: First try Thor's recommendation that a singly loaded cartridge should leave the hold open engaged every time. To fine tune, place a double piece of masking tape across the rear of the frame. In a perfect world the tail of the rear toggle link will dent the masking tape without cutting it. There is probably too much shot to shot variation to do this every time, but you get the idea.
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