getting 5gal pots together, how much water to use?

im getting some 5gal pots ready... how much water should be used to get the soil just right? the soil has been sitting around for a while so it is dryer than what it was when i bought it (foxfarm)

i jus dont want to add to much water to the soil

A gallon should be good, but its always best to water it and then wait like 10 min and see if it comes out the bottom. If not continue adding water till it does