Whitworth bolt's head size

[Henry Harris]

I'll see if I can find my 1961 Technical Drawing O Level notes later, but I remember the formula taught as being 1 1/4 D + 1/4 inch. 

So for instance, 1/4 inch D x 1 1/4 + 1/4 inch  =  0.25 x 1.25 + 0.25  =  0.5625"  =  9/16"

However, checking original A7 nuts, it's oversize!