You want open center. As for cu/in per rec, figure out the cubic inches in your cyl and then divide by the cu/in per revolution of steering wheel and get your turn lock to lock. will be different rod side to piston side on a single ended ram as the rod removes volume from that chamber.
pi X R^2 is square inches of piston then for cubic inches, X length of travel you use in steering (if you don't use all the ram)
So, 2x8" ram, 1x1x3.14 is 3.14 x 8 is 25ish cu/in. so a 9 cu/in orbital for example would be 25/9= 2.777 turns lock to lock.
Open center means the fluid is constantly running a loop from pump to pressure port, out return port back to tank. Closed center means the pressure dead heads and sits, when you turn it gives fluid to the ram, and the other side of the ram exhausts to the return or Tank hose