My main point is that I think this is a flawed approach. I know Sandy teaches it because it is easy for new people to use because there isn't a need to find the ink limits and partitioning points, but Using Curve_K as the only means of controlling the ink density removes several options for getting closer to a working profile.
Also, after installing the profile below and examplining the quad curves, I think the method you using with a single partition and very low ink limits means you do not have the ink in the curve to shift it over to the shadow end of the scale. Assuming you used the spreadsheet correctly (see below) that would be the main reason it is printing so dark. The math for the correction curve is correct, but there just isn't enough ink to move around where it needs to be in the curves. I have run into this same issue when I was making my partitioning system these past few weeks. The math is correct in terms of ink levels, but the actual blocking density of the ink isn't there.
So, for that reason alone it would be MUCH MUCH MUCH better to use at least a 2-gray ink partition and then do the copy curve with all the other inks if that is the way you want to go.
About using the spreadsheet correctly: I don't know exactly what profile you used to print the 21-step target you entered into the spreadsheet, but if there was already a setting in the Curve_K= and then you overwrote it with the one from the spreadsheet then you are just chasing your tail.
Here is what you should do: copy the ink descriptor file settings below to a new text file and install it (if you want to keep on with the single partition and use copy curve)
Then print the 21 -step target
measure the print densities and enter them into the spreadsheet tool
DO NOT CHANGE ANYTHING else in the ink descriptor file copy the curve points the the GRAY_CURVE= setting and save it as a different file name (i use Original-File-Name-CC.txt or Original-File-Name-GC.txt) so you do not overwrite the original quad curves
install the profile and print a second 21-step target with the correction curve and run the measurements through the linearize-data app (just a linearization check)
if the densities are too far off then use the linearize= line from the -out.txt file in a 3rd ink descriptor file and LEAVE ALL THE OTHER SETTINGS ALONE (the linearization will be off if you change ANYTHING)
###
PRINTER=Quad3880
GRAPH_CURVE=YES
N_OF_INKS=8
DEFAULT_INK_LIMIT=100
LIMIT_K=14.2
LIMIT_C=19
LIMIT_M=1
LIMIT_Y=23.75
LIMIT_LC=1
LIMIT_LM=1
LIMIT_LK=33.25
LIMIT_LLK=1
N_OF_GRAY_PARTS=1
GRAY_INK_1=K
GRAY_VAL_1=100
#These settings will allow you to get closer than a Ink_Curve="0:0 100:100"
GRAY_HIGHLIGHT=0
GRAY_SHADOW=3
GRAY_GAMMA=0.8
# Leave gray curve blank and install the profile. Print and measure the 21-step file, insert the measurements into the spreadsheet, and paste the correction curve into the gray curve setting here
GRAY_CURVE=
# No Curve_K setting-- this is being done with the gray partition
COPY_CURVE_C=K
COPY_CURVE_M=K
COPY_CURVE_Y=K
COPY_CURVE_LC=K
COPY_CURVE_LM=K
COPY_CURVE_LK=K
COPY_CURVE_LLK=K
###