Is a BJT at the edge of saturation when Vce is at a given constant or when Vce=Vbe?
Saturation occurs when increasing base current does not reduce collector voltage.
as such it s depedant to some extent on what the load is,

Depending on the degree of precision needed and what you're trying to do 
you can use different approximations for saturation voltage.

For some applications you want to cause saturation to minimize heating, in others you want to avoid it to maximize speed.