Before an Exposure Calculator
You have to find an acceptable exposure before you can use an exposure calculator that brackets that exposure with 2 steps under and 2 steps over exposure, so you can judge the fineness of line you can print.
The minimum exposure to cross link emulsion so water won't dissolve it and it holds in the mesh, is a Solid Step 7. More exposure will make the stencil more durable (harder), but light scatter in the mesh and diffused light from a longer exposure, can under cut the positive, and choke the final image size.
UV energy from any lamp, but especially from banks of fluorescent lamps comes at your fine line positive from the left and right. It hardens your stencil at an angle which chokes the size of your printed dot.
The problem shows up with halftones and fine lines under 0.030". It may turn out you have to make a line 0.040" to that you actually print a line 0.030" and you have couquered Mother Nature's attempt to frustrate you.
Don't under expose, so you can hang on to fine lines that close up, because of undercutting, because you will have a stencil that is not completely cross linked and will break down on the press. You must outwit Mother Nature and calibrate all halftones and finelines to compensate for this choking or stencil failure
ADD CONTROLS TO YOUR POSITIVE
Add simple lines of a known width to your positive and use that positive to MEASURE the result on your final screen print.
Large block letters - Who cares?
The right side was exposed to more UV energy than the left. Notice how the lines are choked.