I agree that the PO4 demand is underestimated normally - but it is not the waste concentration in the water column that is important - it is the flux of PO4 that´s important. You can have 0 in the water and have - as I have - a flux of 0.06 PO4 a day. For safety reason - I aim at 0.04 - 0.06 ppm as waste concentration. (Waste - is not used as a resource for the moment)
How my tank looks like - latest FTS - a month ago