How to bevel a flat surface? Is it possible to bevel a flat surface using the bevel tool?

Here is my object (a simple ramp):


and I want to curve this ramp object but the bevel tool doesn't work on flat surfaces:

[ the bevel tool only create new edges without curves :( ]

The way that I make the curve on the other object has no accurate measurements:



I set the edge bevel weight that be used in bevel modifier;
Configure the bevel modifier;
I try to position the edge in the exact centre making the other
adjacent edges the same length.


So, I wanted to use the bevel modifier to avoid manually manipulating the edge position, because I can't position it perfectly.
ok, as @metaphor_set said, makes sense that bevel tool wont work in flat surfaces


  It still needs an angle, because otherwise a bevel doesn't make any sense


so, i go manually:



this measure addon helps a lot (edit>preferences>addons>"measureit")

[Edit] I found a new way to do this with exactly precision:

I used an plane as a knife to obtain a vertex at the intersection of the edge that I wanted to snap and now I can use the bevel tool. Here is the workflow:


[Edit 2] I found a faster way:

Using "Connect Vertex Path" (Ctrl+J) automatically create a new vertex at the intersection of the created edge. Here is the workflow: