The thing about u-235 is that it's not really about sustained production. It's about production bursts and you want it to recover quickly to have the uranium available for the next burst in demand.
Maybe I want to build up a new mine. To clear the area, I decide to take 30 nukes. The factory will automatically replenish stocks, creating a sudden demand for 900 u-235. In my current factory, which produces 70 u-235/min, that's going to take just over 12 minutes to supply.
Once I clear the area, I will build a train station, and the train station will need nuclear fuel. There would be less overhead if I used a logistics network, but I like belts. To fill one side of the refeulling belt system, it's going to take a couple hundred nuclear fuels, which is even more demand on the system.
At 70 u-235/min, let's call it an even half an hour to fill demand.
Is that responsive enough for my needs? Well, it seems to be alright, but now that I've worked it out, I think I might improve it.
Come to think of it, even my reactors have a burst sort of demand on u-235 as well. The nuclear fuel cells are delivered to the reactors by the trainload. Each train holds 2000 fuel cells so that's an instantaneous demand of 200 u-235.