Once you have GHA bloom in your tank, PO4 is most likely to be zero. These GHA will just soak up all the PO4.
PO4 does not just enter the tank through your water, fish food has plenty of PO4 too. RO will help but it does not guarantee no GHA in the future.
Even if you don't have a refugium, as long as you keep a few turbo snails, the GHA will absorb PO4, and as soon as it starts to grow, the snails will come and chew it away. This is how the Eco system works.
So my advice is don't worry too much about hair algae, it's part of your system, and it's very good at removing nutrients. Just don't let it grow too wild, let the turbo snails keep the GHA in check.