My requirement to dose phosphate was borne of the need to get rid of dinoflagellates. When setting up my new tank both nitrate and phosphate hit zero. Nitrates climbed eventually but nothing helped to raise phosphate, and dino continued to plague my tank.
I turned to dosing Tri-Sodium Phosphate.
Over a period of two days my PO4 went from 0ppm to 0.12ppm (which I specifically aimed for). I did double the solution mix in order to increase the dosed amount, making the 1L solution go further.
I tend to find that in my tanks, the doubled solution raises PO4 by 0.01ppm/13ml dosed.