Hello Mel, the idea of using this mathematical expressions is to transform our Radius values into a set of numbers that proportionally add up all together the number 1 so each one of this values represent a segment of out curve domain.You can think of a logarithmic function as the inverse operation of the exponentiation EX: logarithm of 1000 to base 10 (log10 1000) is 3, because 10 to the power 3 is 1000. So what we are doing here is to evaluate all our radius values to the same logaritmic function to retrieve the fractional values of them in this way when we add them all up togheter and divide them we get a fair distribution.For example if we didn´t use the log function and take the radius values 121,8,7,88,11 an add them all up together you get the number 235 if we then divide our radius values with this number (121/235),(8/235),(7/235),(88/235),(11/235) you will get the results .5148, .0340, .0297, .3744, .0468 wich will actually also add up to 1 but using this values as curves domains will not work as for example the value .5148 almost acounts for more than half of the curve, this is because the difference between the radius 121 and 8 for example is huge so evaluating all our values with the same logaritmic funcion gives us a proportinal fractional value we can use to get a proper distribution.I hope this explanation helps you a bit.
