I came across this post on another forum that I modified.
dv = 39.37 (fb x Vd)^1/2
dv= minimum port diameter of vent in inches
fb= box tuning frequency in hertz
vd= cone displacement in cubic meters (one way xmax in meters times cone area in meters)
for one BTL Neo 15
xmax= .034 meters
sd= .018 meters^2
tuning= 32 hertz (per you)
dv= 39.37 (32 x .034 x .081) ^1/2
dv= 39.37 (.088128)^1/2
dv= 39.37 (.29686)
dv= 11.68737 inch diameter port per sub
dv= 107.28 square inches
for two 15's you need (2)dv. So you need 214.56 square inches of port.
Is that correct?