You can use the scanner to arrive at the density values needed for my spreadsheet tool.
The values will be similar to what you will get from a photospectometer (at least in terms of the format, but is still dependent on what you get from the scanner and how it is calibrated). If you are using a scanner for this process the best you can hope for is "close enough" anyway.
First, scan the target with no scanner adjustments and load it into Photoshop. DO NOT adjust levels for the black and white point (which is contrary to what most people say to do). You actually want to be able to measure what the paper white is and changing the output black and what point will distort those measurements).
Set the secondary color readout in the Info Panel to 32-bit L*a*b*. Set the Eye Dropper sample point to 11-31 px (if there is a lot of grain/noise in the scan you can select and average blur each patch). Starting with white, read the measured value and type it into the spreadsheet I linked to below.
I have a tool that allows you to compute density from L*a*b* values here: https://www.dropbox.com/s/ydx9q075ojulpqb/BWMASTERY%2021-step%20Lab%20to%20Density%20Conversion%20Tool.xlsx?dl=0
Then use the correction curve tool to arrive at the line to paste into the ink descriptor file. (f you are using the K_CURVE= Line paste it there, otherwise paste it into the GRAY_CURVE= line.