Printer lites are pretty straight forward. The average of the numbers represent the density. When printing from negative the timer increases the value of the color that is to be taken out of the print. Creative timers manipulate all three numbers to get the results they want, but the principle remains the same. The numbers that differ from the normal value of 20 represent changes in color and their average represent the density.
Printers have three independently controlled lite valves.
Printer lites correlate with these shutter positions. Not all laboratories use lite 20 as their normal lite. Some use 25 and others 30. Before one tries to interpret the printer lites they should ask what the printing laboratory considers normal. A normal lite 30 at a lab that uses 30 should yield the same results as a normal lite 20 at a lab that uses lite 20 as their normal.