How far a marble top should overhang the frame beneath it is not a number anyone can give you, and treating it as one is why some marble-topped consoles look slightly wrong without the viewer being able to say why.
Too little overhang and the carved frame appears to be bursting out from under the stone. Too much and the top reads as a slab that happens to be resting there - borrowed, not built in.
Our master Indian artisans set it by shadow instead. Under any overhanging top there is a dark line where the stone shades the frame, and that line is what the eye actually reads. The overhang is adjusted until the shadow band is even in width all the way round the shaped front - which, because the front is serpentine and the light is not, means the projection is not constant. It is slightly greater where the front swells forward and slightly less where it returns.
The moulded edge of the stone is then profiled to sit in the same relationship to that shadow. The result is a top that looks grown out of the frame rather than laid on top of it.