We need more info.

What row spacing?

What GPA will you appy?

Formula = GPA x MPH x Row width / 5940   (magic number  ) x [conversion factor for density]

(Your conversion factor should be 1.16) = I got conversion factor off of Wilger flow charts, setting beside me.

Then you have to look at chart (charts are based on water)

http://agrimart.net/0-149-gpm-orifice-plate/

{Edit: I'll guess on row spacing and GPA, in these examples}

One example:

(5 GPA x 6 MPH x 30" row) divide by 5940 (magic number) x 1.16 (conversion factor) = .17575 ; then on charts for a #37 = 42 psi

2nd example:

(3 GPA x 6 MPH x 30" row) divided by 5940 x 1.16 = .10545 ; then on charts for #37 = 15.5 psi