If you want to calculate precisely the exposure time needed for your setup, you will need to use an exposure calculator.
Here is how you can use it :
Start by deciding the time interval you want to test (in seconds). For our exposure setup, we usually go for 30 seconds intervals.
Print our exposure calculator on a transparent and set up your exposure setup. Your film master needs to be flat on the mesh of your screen, using transparent tape or plexiglas.
Prepare or thick paper or cardboard that can block the light.
Block all but the last section of the calculator, and turn the light on.
After the time interval you've decided to go for, slide the cardboard to expose the second to last section. This will expose your second to last section for the time interval selected, and the last section for two times the time interval.
Repeat the operation until all sections have been exposed. If you have 10 sections of 30 seconds, your last section will have been exposed for 300 seconds.
Turn the light off.
Reveal your screen with water the same way you would do to prepare a screen.
Make a print on the same support you're trying to print on in order to decide what is the best timing for your setup.