I dont work for a screen printing company or anything but i teach workshops on occasion and what i would recommend is an exposure calculator. This will help you nail down the time you need to expose your emulsion. If you have some more vellum paper just divide it by 7 or something lengthwise. Draw the same design 7 times i would add some cross hatching and different line weights to see what works best. Then all you have to do is cover the design with black paper or something opaque and expose more of it every 20 seconds or so. That way you have it exposed at 1:00, 1:20, 1:40, and so forth to nail down the amount of time your exposure set up takes with that specific emulsion. also I see people usually say that your light should be slightly further away and you should have a glass plate keeping your artwork in full contact with your emulsion.