The thickness of the stencil determines the proper exposure necessary to completely harden the emulsion from top to bottom. This is exacerbated by fact that each mesh count/thread diameter combination produces a different emulsion thickness due the volume of the mesh opening. As well, no two screens, even those of the same mesh count and thread diameter will have the same stencil thickness. This is a result of different angles of the screen during the coating process, the speed as which the scoop coater is moved, and the rheology of the emulsion, which varies with temperature.
The use of a exposure calculation film or using a step wedge will only give you the correct exposure on that one stencil.
By measuring the stencil thickness with a electronic thickness gauge, you can determine the necessary exposure time prior to exposure. By keeping a log of the correct exposure based on stencil thickness to achieve opacity on a particular mesh count/thread diameter, you can reduce the need for double strokes and increase production.