Standard recommended wash solution dilution ratio is 256:1 which you are effectively at (58.6 ml to 15 L to be precise). Since you are using Reverse Osmosis water there should be no contaminants and I would stick with this dilution ratio . In my case, I live in a region with moderately hard water (260 ppm), so I will typically use a slightly higher dilution ratio (around 225:1) to compensate for the ONR attacking the minerals within the water.
For your pre-spray solution, you may want to consider a higher dilution ratio depending on how you use it (apply and rinse with clean water before using BRS) and level of dirt/contaminants on the vehicle. In the past, I have used as high as 64:1 dilution ratio especially in winter when there is excessive salt build up on the panels. But if the current 256:1 dilution ratio works for your conditions, I would stick with it.