With a little more information, someone might be able to give you something to try.  What are the settings you are using for this?  Also, it would help if you could provide the numbers at the bottom of the screen.  Perhaps a screenshot of the whole window would help, as that would show the settings and the numbers.  Once you are more familiar with LuxRender, those numbers can help you decide what to change and how. 

 

As others have said, there aren't any magic formulas.  Each render will require some fiddling with the settings.  I will say that I use linear exclusively; but that is because I am a professional photographer and that is what i am most at home with.  Others may use other kernals in the tonemapper.   If you are using the linear tonemapper, don't hesitate to use the estimate settings button.  It may not be perfect, but it is usually pretty good.