Melev, having used it with a quality po4 test kit - I can tell you my reco - and that would be not to follow any "guidelines". Simply get a colormeter or Deltec/Merck kit and start out with 200-300 ml of GFO.
Make sure to rinse it well, both Doug from Deltec and SeanT have posted good info. IMO, its not the drop in po4 that hurts corals, its probably the dust that is burning them - possibly the iron. Depending on how dusty or small the GFO you get is, you might want to but a couple more Phosban sponges to make sure you keep it in - or just buy an aquarium sponge somewhere and cut it to shape.
But there is no rush to take out the po4 overnight - just put ~250ml in the reactor and run it for a few days then toss it (since GFO is so cheap). You can measure the discharge water - that is what you're supposed to do - but 250ml won't make a dent in .1 po4 in a 280G tank - and tests are expensive. So maybe just use 250ml for a few days, toss, refill .... then test the water and effluent after a few times.
But any GFO product is probably going to vary so IMO rules don't apply. Just be safe and go slow.
JMVVVHO!