Your weight (that is You and the weight of your bike & equipment) is a prime determinant for setting correct tire pressure, as the tires are carrying it all. Since you are riding on the road you want to keep the tires pumped a bit former than if you ride on dirt or gravel.
You want the pressure to be 10% more in the rear tire than in front. The absolute pressure number is also determined by the width of the tires; more for narrow than for the wider ones, as the higher pressure compensates for the lesser volume of air.

If you weigh less than 200 lbs you might want to start with a pressure of 50 lbs rear and 45 lbs front; then raise the pressure in 5 lb increments if you're feeling too much drag while riding. I like to keep mine a bit soft in order to absorb the bumps in the road.

Because of the risk of pinch flats it probably wouldn't be a good idea to go any lower than the 45 lbs they recommend.