Logarithm, exponential generator

So are there any George Boole's out there? (or at least someone who's more learned in the dark art of mathematics than me).
I'm making a roll generating part of a bigger Max patch.
Let's say (for the sake of ease) that all I want to do is generate a roll on the last 8th of the bar.
To get the length of that 8th in ms would be BPM -> [!/ 60000] -> [*0.5] = 1/8th.
Say I want the number of beats in the roll to be 5.
At 120BPM the 1/8th notes would equal 250ms so evenly distributed, each beat of the roll would be 50ms. But I don't want even distribution.
I want to achieve a less robotic/metronomic roll, that is either logarithmic or exponential.
I could probably do this in a really convoluted fashion with loads of different objects. Using the above example;
1/8th = 250 -> [ /5 ] {amount of beats in roll} = 50.
a-20% b-40% c=50 d+40 e+20% = 40 45 50 55 60
I just haven't got a clue how to express this as there are many variables.
I would like to be able to alter the amount of beats in the roll from say 2 - 8.
Then I would like to randomise the exponential or logarithmic curves.
If someone can give me a basic grasp of the equations and how I may achieve it with [expr] or something then I'd be extremely grateful & of course owe you a pint. I am actually nearly there I think.