Your best bet is to check the manufacturer's web site and follow their recommendations. If that's not possible, the next best thing is probably a test board. Just make a long thin board with a test pattern on it, and draw a bunch of lines across it to mark the different exposure times. Then cover the board up to the last line with a card and start the exposure. Every few minutes, move the card to the next line. After developing and etching, use whatever time that gave you the best results. That's my plan anyways... I'm just waiting on supplies to arrive to begin testing the photomask method of making boards.