Percentage coverage is easily measurable using a commercially available reflection densitometer. The instrument only needs calibrating with the null (printing substrate) and full tone references, then it can be switched to measure tonal values.
Should an instrument be incapable of automatic conversion, the formula is:

A process line traces the variance in tone values, plotted as a curve, between those on the film (or output device) and those in the printed image. The screen printing process line allows a repro expert to identify the dot gain (positive or negative), in order to calculate a compensation guideline for the printed image.
The screen printer initially needs no measuring instrument. A half-tone step wedge with at least 10 tone values should be printed alongside the artwork. The repro specialist uses a transmission densitometer to measure the half-tone values on the positive film, and a reflection densitometer on the print. The results are then compared on a chart.

The variances in tonal values, plotted as a curve, result in the socalled process line: