Different PC monitors almost never show the same colour, but the sample with RGB 75 85 97 looks more similar to a clean and non-weathered FS36118 to my eyes. FS36118 is the same colour used on the upper fuselage of the F-16, but I reckon the colour used on the F-15 being slightly darker and less blueish.
Anwyay, have a look at the image attached: they don't look bad either! I suggest you to enlarge then image as much as possible and see how the black codes stand over the grey surface (they look lighter, but they are the same shades you used!).
As well as in modelling, I think there is no strict interpretation whatsoever. Two modellers will never paint the same model the same way and will usually fade the base colour in the middle of the panels (or alternatively will darken the panel lines), and perhaps will recreate dust and dirt effects in order to break the monotony of a plain grey surface and make the model interesting. They will definitely use different paint brands, and no brand makes the same shade, and will add a bit of white to it because painting scale models requires the base colour to be lighter.
So it just depends on your personal choice and which degree of weather you want to depict.
I think the lighter colour could fit to a more weathered airframe, whereas the darker shade could be useful to depict an aircraft that has been painted recently.
BTW, I've taken a look around google and here you can find alternate RGB values
http://www.color-swatches.com/testors/g ... watch.html
(sorry) and like Chris says, I think it is a case of if it looks right.....there are so many greys out there it is stoopid, I don't think it'll ever be a case of "that is exactly the right colour" 
