Use a real transmission step wedge in contact with the stencil. A grayscale pattern printed by an uncalibrated office printer is not automatically equivalent.
Record the solid step after normal processing
Expose once at a known time, wash out using your normal method, dry the screen, and read the last step that meets the criterion supplied with your wedge and emulsion process. Record both the time and the observed step.
Use your process target
Do not assume every stencil should hold the same numbered step. Step 7 is a common direct-emulsion example, but other stencil systems and qualified shop procedures may use another target. Enter the target supplied by the emulsion maker, wedge instructions, or the quality procedure your shop has already qualified.
On the common 21-step scale, each step represents a half-stop change. Moving one step higher requires about 1.41 times the exposure; two steps require 2 times; moving one step lower uses about 0.71 times.
The correction formula
corrected time = current time × 2^((target step − observed step) / 2)
If 30 seconds holds step 5 and the qualified target is step 7, the difference is two steps and the corrected starting time is 60 seconds. Run a confirmation exposure before treating that number as production-ready.
If the factor is greater than 4× or less than 0.25×, treat the calculated number only as evidence that the first test was too far from the target. Run a new bracketed test around a closer base time instead of making one extreme correction.
Know what the wedge cannot diagnose alone
A wedge can show exposure response, but it cannot prove that the film is dense enough, contact is sharp, coating is uniform, washout is consistent, or fine halftones will print correctly. Evaluate resolution and stencil hardness together.
Primary factor reference: Stouffer, Using the 21-Step Guide
Process reference: Ulano ExpoCheck