light system

I’m studing a light system for a simple cad 3d.
I use a three light system(fill,key,back light).
My problem now is that i have some discontinuity in the illumination:
sometimes in some position the light factor is low.
I have two option :
1)insert another light with some compute increment
2)I’m not using the ambient light, is possible to resolve these “light bug” adding an ambient light?
3)add a multiplier factor for the diffuse light and adding an ambient light(.2)?

If i add the ambient light ,how must be intense?
there are some simple equation or a simple system for calculate how much ambient light i must add or i must test it “at hand”?


Classic ambient light from 0% to 30% had been seen, it all depends on how you want your lighting.

A screenshot showing your issues would help.