depends on how many Gallons a minute you have available and what the precipitation rate of the sprinklers are and how much loss there is in the system.
70 psi will not be 70 psi at the sprinkler heads, you must calculate the pressure loss in the pipes. Loss depends how big and how long the pipes are, the valves you are using, backflow you are using, loss of the meter, etc....
http://www.irrigationtutorials.com/sprinkler00.htm