Having looked at the 05 and 11 rules there is some different around calculating the F + G values, the 05 rules uses difference in leach / luff and 11 uses difference in leach and foot? Which of these is correct? Turns out my formula follows the 11 rules formula. For those who don't mind badly written javascript see below:
var lMean = (+leach + +luff) / 2;
var footPower = Math.pow(foot, 2);
var llPower = Math.pow((leach - foot), 2);
var f = Math.sqrt(footPower - llPower);
var g = Math.sqrt(Math.pow(mid, 2) - Math.pow((leach - foot) / 2, 2));
var lMeanF = lMean * f;
var a = (lMeanF / 6) + ((2 * lMean * g) / 3);