Digital BW, The Print

Hi all, need some help again please.

I have been trying to set up QTR and have got as far as the linearize
curve. I recently had to re-install my Epson 4870 scanner and when
setting it up again I noticed a densitometer tool in the scan window.
It has its own thumb size target/preview and before &  after RGB and
Luminance values to the left of the preview. Under it are two more
values in parentheses. This pair of numbers can each have a value of 
numbers that can be any combination from 0 to around 680 or so, but
I'm not sure what they mean. If any light could be shed on this I'd be
grateful. 
So which value do I use to linearize a curve in QTR. I guess its the
Luminance value? 

Thanks.

Mark.