Ok well you need to start dosing nitrate straight away, dose 1ppm no3 today, 2ppm no3 tomorrow and keep upping the dosage by 1ppm each day until your test kit gives you a reading of 5ppm or above and then start dosing 5ppm each day until you get a reading of 15ppm.
Basically that will probably take you about a week, 15ppm is your target
When it comes time to do your weekly water change dose 10ppm no3 to the tank, this 10ppm plus whatever is left in the tank will bring your levels to around 15ppm and then your job during the week is to figure out how much the no3 levels are reducing as the plants consume it, and you basically top it up every couple of days to bring the level back to 15ppm.
So firstly get the no3 levels up to 15ppm, do the weekly water change, and dose 10ppm to the tank and then during the week do tests for nitrate and try and work out the uptake rate, and do topups of no3 as the plants use it up to maintain a level of 15ppm approx.
You dont have to be that accurate the guidelines are pretty wide, when i say 15ppm its ok to be a bit over or under, this is the estimative part of EI, the important thing is to not let anything run out.
In my guide i say to try and reach a target of 10ppm but on reflection this is probably a bit low for people starting off on EI, it doesnt leave enough room for error, 15ppm is a good level especially with higher levels of phosphate, its a bit more balanced.
The first week is to get the no3 levels up and then next week you can start EI proper when you have reached the target level.
Hope that makes sense
Edit: id say with the amount of PO4 in your tapwater you would want to maintain no3 levels at between 15-20ppm i would try not to let it drop too far below 15ppm no3.