Duncan, where did you get that formula from? 😀 Never seen anything like it. Metric is ((2nd-1st)*3600*10.76)/time in gross so divide 1.11. Imperial (1040*3600)/3412/time, again divide 1.11. If its new boiler, badge will be in net.
It seems that everybody is doing it different way.
I got that formula when I was in College. They called them magic numbers. I also know the way you have showen. However, I find these easier to remember. 😀