yes mate, thats right. It doesn't matter weather you are in veg or flowering stage, the weekly flush is to remove salt biuldup in the solution/medium and remove wastes from the rootzone. Deviant explains what happens in the TDS link in my sig. It's a bit of a guess which nutrients the plants have actually used up (the only way to tell acuratly is to send plant matter to a lab and compare nute concentrations). The plant can also evaporate water from the res faster than it uses nutes, this is how the solution becomes too concentrated and becomes toxic.
By flushing with 400ppm nutes, and doing another nute change afterwards, to the desired level for your stage of growth, you are stressing the root system less than you would be if you used plain old water(somthing to do with ions exchange in the rootzone). Canging nutes removes the uncertainty of what is making up your nute solution after the plant used only some of the nutes you are replacing during top ups. Removing waste material and preventing salt biuldup is a weekly affair, a new nute solution is a useful side effect
Have a read of the hydro links in my sig for advanced insight
Sanster
