One of the better ways to do a test is to do a step wedge.
Basically, you will have several exposures on the same screen. A quick method would be to print a solid block, 1"x1" and go the length of an 8 1/2" x 11" paper. Put a little space in between. Tape it to the bottom of the screen. Now take a large sheet of black or heavy paper that light cannot go through and cover all but one square. Expose for one minute. Slide the paper to expose 2 squares, expose for one minute.... etc. etc. until all the squares are exposed. You can do this with halftones and solid squares as well for a more accurate burn time. You don't have to start out at 1 minute, but maybe 3 minutes and 7 boxes all the way up to 10.
Then wash out the pattern and see which washes out the best and you have dialed in your exposure time.
You also will have a difference in times with what mesh you are using, if it is white or yellow and how many times you coat your screens. So you really need to test to get the right times.