I have some phosphoric and plan to do this on my next lighter brew still looking up the calculations myself, I ran into the same 80% issue, but we should just be able to add 8 times as much 10% solution I'd imagine.
"People should not be afraid of their governments. Governments should be afraid of their people." - V
Secondary: Shady Lord RIS, Water to Barleywine, Pumpkin wine, burnt mead
Kegged: Crappy infected mild
Bottles: Apfelwein, 999 Barleywine, Oatmeal Stout, Robust Porter, Robust smoked porter, Simcoe Smash