Modifications to a sourdough recipe
Good question! Yes, if you use a 50/50 mix to feed your starter then your approach will give you the same balance and you will get the same hydration, providing you use the same flour blend as when you feed your starter. If you use a different ratio to feed your starter then stick to whatever ratio that is.
This approach will slow down your proofing for sure, maybe more than you intend. It's not linear, as in you want to slow down your proofing by 2/3 so you remove 2/3 of your starter,  so do this when you have lots of time to study it. Take detailed notes and see how you get.
Another approach to slowing fermentation is to control for temperature, i.e. to put it in the refrigerator. I'd prefer this approach to reducing the starter as I'll be starting with a good batch of yeasts and giving them more time to work. Reducing the yeast at the start will mean it takes longer to get going, but also a lot longer to see the benefits of that culture.