// Calculate height
$h = 3.5938 * log([D:m]) + 1.6936;
// Return nearest half integer value
return floor($h * 2) / 2;

Approximates data in Table 3.3 by a logarithmic function (R² = 0.77).

Original diameter range is 2.8 - 45.72 m (9 - 150 ft).