How do I calculate resistance valve given the flowing information

F$1=1$ ft$^3/$min$,$ h$1=$ h$2=3$ ft and crossection area is $5$ft$^2$ how did I determine the valve i know one method is Cv$=$ Q$($F$1)*$ specific gravity$($ so I would have to make an assumption$)*$ pressure drop. The tank is open and it is going from tank $1$ to tank $2$ I