Here is a diagram of the fork and damper rod:
When extended the fork tube is all the way to the right with the end plug seal holder against the rebound spring. The space between the slider and damper tube is the compression volume and the space between the inside of the fork tube and outside of the damper tube is the rebound volume.
As the fork moves down the oil goes through the slots and two holes into the damper tube and up through the holes in the damper head nut then down through the damper into the rebound volume.
When the top of the end plug moves past the holes there is a drop in damping because some oil flows directly into the rebound volume bypassing the damper head.
When the end plug moves down so the slots open above the top of the end plug the damping almost goes to zero because there is very little restriction in the flow path.
When the end plug gets to the bottom of the slots it serves as a hydraulic bump stop except for the leakage past the end plug and damper tube.
On the way up all this reverses except the washer with the four holes under the damper head is pushed up against the damper head and increases the damping on rebound (I probably got this backward in my earlier post).
The damping curve looks like this:
I do not remember what viscosity that I used for the calculation.
My recommendation, instead of shortening the springs and adding a spacer between the topping spring and end plug was to make a shorter damper tube which would move the slider up the fork, keeping the distance to the fork top nut the same and lowering the front end.
Instead of the slots, put four holes near the bottom of the stroke on the damper tube, two at the top of the lower two. This will force the oil to go through the damper head both ways. When the first set of holes are covered by the end plug there will be an increase in damping and a hydraulic stop when the lower holes are covered.
You can replace the damper head with one like this:
The nut has a series of holes stepping down with a socket head screw inside. As the screw is moves up and down the overall damping can be changed externally through the top nut.
The shim stacks under the nut control the compression and rebound damping by diameter and thickness of the shims.
The only thing missing here is a "blow off" spring for the compression. You would have to add a wave spring under the lower set of shims holding them against the damper head but letting them move off on a hard hit.