The Formula is:

Q = C * √ΔP

Q = desired flow
C = valve flow rate (from table below)
ΔP = psi

Example:
For a 3" valve  C = 170 gpm at  1 psi

Let's say that you want to know what the flow rate is at
a pressure of:
Δp = 0.85
and
Δp = 3


For the first psi of 0.85:

Q = 170 *√0.85 = 156.74 gpm

For the second psi of 3:

Q = 170 *√3 = 294.45 gpm


Asepco Valve Cv's (at 1 psi):

0.5"	02.8 gallons per minute (gpm)	10.6 liters per minute (lpm)
1.0"	15.8 gallons per minute (gpm)	60 liters per minute (lpm)
1.5"	32.0 gallons per minute (gpm)	121 liters per minute (lpm)
2.0"	72.0 gallons per minute (gpm)	273 liters per minute (lpm)
3.0"	170 gallons per minute (gpm	643 liters per minute (lpm)
4.0"	302 gallons per minute (gpm)	1143 liters per minute (lpm)