I would say that Tom is about right on an estimated uptake, based on PO4.
My tap water PO4 is about 5 (dark blue on the AP kit). My tap NO3 is around 10 so I add enough until it gets to about 15-20 ppm. This is on Sundays after water change.
On Tuesday morning I'll test PO4 before I head to work and not much of a difference, except the PO4 is a tad bit lighter...maybe down to 3ish ppm. NO3 is about 15.
By Thursday my PO4 is around 1-2 and NO3 back to 10. So that's the only other time I dose macros to push my NO3 back to 15-20 and my PO4 to 2-3.
But then again, I'm convinced this uptake is based on the types of plants I have. I do have a couple large swords, but the majority of my uptake is probably through my giant hedge of wisteria, my P. stellata stand and stargrass.