Make a test strip to check the exposure to your source. Mask half the test strip left - right and then stop the strip in 1cm slices from top to bottom. Expose the panel with the first 1cm covered - try 2 mninutes. Then cover the first 2cm and expose for another 2 minutes. Keep covering/ exposing until you run out of strip.

Develop to the makers instructions: see which panel on the board has come clean at the left/right edge best: that give you your exposure time.

In our UV box, exposure is 20 seconds with precoated boards.

If this is precoated PCB, life is a lot easier, on self coated PCB uniform application has to be done very carefully. Spinning the board on a turntable helps greatly.