The core responsibility of the screen maker is to able to judge exposure time with tests. It takes a lot less time to actually test a screen, than for me to actually type up these notes.
If I was at your shop today, I would still have to make a test.
Diazo sensitizer color change
When you test different exposures (like any cooking process), stencils with diazo sensitizer
will change color during exposure because the yellowish diazo will actually degrade from UV reaction and the stencil will return to the color is was before you sensitized it.
When you don’t see any more color change, all the diazo is used up, the stencil has reached it's saturation point, and more UV energy just doesn't change it any more.
Don’t waste any more time exposing it, if it won't change any more.
If you are using a pre-sensitized emulsion, there is no color change so you can only judge if the stencil survives or not - which is why you want to use a standard Stouffer 21 Step Gray Scale to simulate 21 exposures on every screen you expose for the rest of your life.
Your goal for starting is to have a Solid Step 7 survive the developing process.