I'm trying to figure out whether it's better to capture somewhere close to where I think the levels should be by eye, and risk crushing or blooming detail, or constrain the range so that it definitely fits within the ends of the histogram, for later expansion with the Excel function.

Bars here from Visions of Light. Both captures are using the ATI 600 USB. The first is with high brightness and low contrast (dull image) and the second is with lower brightness and higher contrast. My attempt at approximating the appropriate values didn't go over too well due to the laptop monitor that I was using. For some reason it causes things to look way off.

Visions of Light LD Side 2 Bright130 Cont23:

ColorYUV(gain_y=124.07, off_y=-35.84, gamma_y=-28.07, off_u=-0.16, cont_u=31.19, off_v=0.26, cont_v=63.19)

Visions of Light LD Side 2 Bright107 Cont29:

ColorYUV(gain_y=41.22, off_y=-0.33, gamma_y=-26.67, off_u=0.30, cont_u=36.82, off_v=0.26, cont_v=63.19)

http://s2.postimage.org/ryijc93o9/Vo_L_ATI_600_High_a.png
http://s2.postimage.org/ntc842va1/Vo_L_ATI_600_Low_a.png

http://s2.postimage.org/p5pbs83bt/Vo_L_ATI_600_High_b.png
http://s2.postimage.org/owwcg1fx5/Vo_L_ATI_600_Low_b.png

Output:

The dark blue bar and dark purple patch seem to show the biggest differences. Beyond that I'm only seeing noise shifts. I don't know which is better, but at least I'm satisfied that there isn't a huge difference either way.

Histograms:

http://s14.postimage.org/c2xiunwwx/Vo_L_ATI_600_High_d.png
http://s14.postimage.org/4bgsw3srl/Vo_L_ATI_600_Low_d.png

Looks like my approximation didn't leave enough play at the low end to avoid clipping below-black.