You are giving yourself some good advice! If you think your tap water is a bit high in phosphates, just consider the tap water by itself to be one dosage of phosphate, and dose per EI for the remaining doses before the water change. EI light, as I recall, is for those who have good reasons for not dosing beyond the plants immediate needs - lighter than usual plant mass, lower light intensity, slow growing plants, etc. With as much light as you have you need to be sure you are dosing 30 ppm of CO2, as you have already figured out, but you should also consider switching to normal EI dosing. You are making good progress in any case.