With this things you can easily level your printing bed.
First level it with the paper method and then print one of this calibration objects.
I added circles at the corners, because circles are generally more critical to print when the bed is not leveled right than straight lines.
During printing check if the filament sticks well to the bed. If not calibrate the bed during the priting process. If filament doesn't stick bring the level of the bed a little bit closer to the nozzle, if the nozzle makes only a really thin line of filament level the bed little bit more away from the nozzle.
depends on what you want to test