Most people aim for 10 - 20 ppm of nitrates.
Most people use potassium nitrate only and do not use urea. This is because urea will (eventually) hydrolyze into two molecules of ammonia. As you probably already know, ammonia is toxic to livestock, and hence, urea is generally avoided.
Regarding dosing, there are several methods of dosing such as Estimative Index (EI) and PPS-Pro. For more information, take a look at the "fertilizer" section of my Planted Guide (see the link below in my signature).
For a calculator, Wet's calculator is quite good: