QTR-Quadtone RIP

Hello list,

I would like to learn how to make QTR curves. I made
two tests with 100% and 85% ink limit. Now I want to
do the ink density calibration. Here is the
explanation in the QuadTone RIP User Guide:

*****************************************************
With a densitometer: 

1. measure the 100% Light Black patch: Density 1.29
 
2. measure the 40% Black patch: Density 1.22. I
selected 40% by eye, but you can use the densitometer
to find the darkest patch with density less than the
density of 100% Light Black 

3. Measure the density of the next darker patch \ufffd 45%
Black: Density 1.33. Then it\ufffds bit of simple math to
calculate the percentage of black needed to produce
the same density as 100% Light Black. In this case we
get a value of 43.2% Black is equivalent to 100% Light
Black. 
******************************************************

Could somebody, please, explain to me the "simple
math". I am using UT1 inks and Epson R220 printer.

Thanks in advance.
Kemal