I use a voltage regulator. It's got two terminals marked + and - for the input, and two marked + and - for the output.
Sometimes if I'm scrubbing frames on a first clean or something like that, I turn it down to save on water.
Generally though, as I clean windows at maximum psi, the thing of a flow controller keeping the same flow rate at regardless of height doesn't matter.
As far as I'm concerned and for my way of cleaning windows, flow controllers are a complete waste of money and prone to going wrong.
I've yet to read one single posting about a voltage regulator problem. Even if it did pack up, I'd spend another £6.99 and buy a new one.