// Calculate equivalent TNT mass
$M = [fyE:q] * [Qf:kg] * [Hc:kJ/kg] / 4680;
// Calculate scaled distance 
// The following equation is an interpolant of data obtained from peak overpressure vs scaled distance plot available in TNO's Yellow Book)
$r1 = (1.346663 + (91215300 - 1.346663)/(1 + pow([ePd:kPa]/0.00000002607457,0.8005746)));
// Calculate explosion radius
$r = pow($M, 1/3)*$r1;
return $r;

• U.S. EPA, "Risk Management Program Guidance for Offsite Consequence Analysis", 1999
• C.J.H. van den Bosch, R.A.P.M. Weterings, N.J. Duijm, E.A. Bakkum, W.P.M. Mercx, A.C. van den Berg, W.F.J.M. Engelhard, J.C.A.M. van Doormaal, R.M.M. van Wees, "Methods for the calculation of physical effects due to releases of hazardous materials (liquids and gases) - Yellow Book", 2005

This equation is used when the endpoint overpressure is NOT 1 psi.
See U.S. EPA (1999), Equation C-1 (section C-1, page C-1).
The equation is based on TNT-equivalency method. Original reference is CCPS (1994). Heat of explosion of trinitrotoluene (TNT) is taken as 4680 kJ/kg.
Distance is obtained by multiplication of the cubic foot of the TNT-equivalent mass and the scaled distance.
Scaled distance is calculated using an interpolation formula, calibrated using data that relate the overpressure in kPa and the scaled distance in m/kg^(1/3) taken from the TNO Yellow Book, Chapter 5, Figure 5.6.