its called the Golden Rule, the ratio of 1.61.
It is based upon a mathematical series called the Fibonacci series. the series is the addition of one number with the one previous.
1+1 is 2
2+1 is 3
3+2 is 5
5+3 is 8
8+5 is 13
and it goes on such....
1,1,2,3,5,8,13,21,33,54,87 etc
now the golden rule is that any number in the series divided by the number before it equals aproximately 1.6.
take 21/13 equals 1.615
13/8 equals 1.625
as the numbers larger they all become 1.61 and this is the golden number or ratio.
It is often called the 3-5 rule since these numbers are easy to remember, but do not divide to 1.61 itself, but 1.66....
does that help at all?
P.