// Kinematic viscosity of air (m2/s, at 300K)
$v = 1.568e-5;
// Froude number
$Fr = pow([u:m/s], 2) / (9.80665 * [DP:m]);
// Reynolds number
$Re = [u:m/s] * [DP:m] / $v;
// tan(theta) / cos(theta)
$c = 2/3 * pow($Fr, 1/3) * pow($Re, 0.117);
// Theta (rad)
$theta = asin((pow(4 * pow($c, 2) + 1, 0.5) - 1) / (2 * $c));
return rad2deg($theta);

See equations 6.68 - 6.70 in section 6.5.4.3 (page 6.66).