This enters the realm of formulation chemistry.
So to your question. I assume that generations of chemists (and schoolboys) have investigated this reaction (I know I certainly did!), and found that the ideal formulation was as you specify. In other words this is an experimental result. But to a first approx. 65:35 is not that much different than 17:6.
As a matter of interest, I tried to do this reaction last year with my A2 class. I am still signing COSHH forms and providing risk assessments; my enthusiasm for the experiment has waned a bit.