If we were to find out that our tap water contained every single nutrient needed by the plants in the ppm's that would be achieved by a single EI dosage of ferts, would we just assume there was no need to fertilize at all? Of course we wouldn't if we were doing any serious thinking. The plants use up the ferts as they grow, so we have to continue to dose ferts to replenish them. So, if your tap water has 5 ppm of NO3 and 1 ppm of PO4, just ignore that completely and go ahead and dose per the EI recommendations. At worst you will have a slightly elevated level of ferts - not a problem in any way. If you still are not convinced, then just consider the tap water to be the first of the 3 doses of ferts the plants need for the week. Then after a day, start dosing per the EI recommendations.