Just send G29 J and it'll do a 3x3 probe, calculate a tilted plane and then tilt the entire mesh accordingly. This assumes an existing mesh.
Or... for backward compatibility... You can do a G29 P0 to create a mesh full of 0.00's. And just tilt that like the earlier version of G29 (Linear) still do...
For physically leveling the bed... I do a G29 P1 U T The U option tells it to acquire points far away from the nozzle's starting point. So especially if you start in the center of the bed, you get outside points probed first on your bed. When you have enough points to see what needs to be done, you stop the probing and make adjustments. Most beds are not really flat. So it is a judgment call how to 'level' the bed. The good news is, what ever you decide, when you probe the mesh for real... It works very well.
U Unlevel Perform a probe of the outer perimeter to assist in physically leveling the bed.
It seems that it will only print the information, and will not act on the motors to correct the table. Is that right? I'm talking about doing the physical correction the same as with dual Z endstops, but using the probe instead (you would not even need the whole grid, just two probes on left andd right would be enough to align the bed, and then go on to the real G28 and G29).
The problem is that the motors dis align too easily when motors are off. Trying to remove the print from the glass is enough to do it. So the idea is to re-align it every new print, not just once manually.