So instead of 6mm longer, cut some that are 15mm longer, is that correct?
Having trouble with one of the forks, the bolt at the bottom just turns around but doesn't unscrew, so it seems the piece in the damper rod is turning. I may have to drill it out and find another bolt somewhere.
Because the original amount of installed preload is an unknown, the combination of stock spring and spacer length is of no insight or use.
So, to do it right, you're going to have to do some measurements, a calculation, and then the cutting.
In conceptual terms, imagine the fork fully extended with the spring in it and whatever washers you intend to install sitting on top of the spring.
The top surface of the top washer will be well down from the end face of the fork tube, as you already know.
Then imagine how many mm down from the end of the fork tube, the spacer’s top perch surface will be when the fork cap is fully threaded in. (for adjustable 04+ forks, do this with the preload adjuster fully backed out so all the indicator rings are showing up above)
One can easily visualize by looking at the parts, that there will be a gap between the above two described surfaces.
That gap would be the spacer length needed for zero preload, so making the spacer another 15 mm longer will net 15 mm of installed preload.
In as simple terms as possible, here’s how to do it for 919 forks:
Fit a spring, plus all the washers
Pull the fork tube as hard as you can.
Measure the distance from the end of the fork tube down to the top of the stack.
Write it down as “A = xyz mm ”
(04+s have a perch cup that sits on top of the spacer so for 04+s, put the perch on top of the washers)
Look at bottom end of your adjustable fork cap.
Determine the surface of the fork cap that the top of the spacer will butt up against.
Measure from the bottom of the fork cap flange down to that point.
Write it down as “B = cd mm”
Determine zero preload spacer length by A – B = Zs
For what ever amount of Installed Preload in mm that you want, simply add it to Zs.
Lucky for you, the 919 has extremely stiff and short top out springs (internal type), so they can be ignored.
Otherwise the above would have to be amended to reflect top out spring allowable counter growth.
As a self check, or even to just maintain confidence at any step, go back and revisit the conceptual sketch above.
As long as what you are doing is consistent with the conceptual sketch, you know you are on the right path.