Hi BC,
Interesting subject and not one I know a lot about!
What you are describing (i.e. area of analyte peak/IS from chromatogram) sounds more like a conventional internal standard calibration curve as opposed IDMS which (I would suspect) would plot something like isotope ratio (of H vs D) against ratio (H vs D) in the spike. Again, I know little of this subject and may have misunderstood your post, so I may well be very wrong but I would imagine isotope ratio (as opposed to chromatographic peak area) should be part of the plot.
A conventional IS curve should in theory produce a linear response throughout the analytical range and in an ideal world should have an R2 of one and intercept of zero! Of course that never happens and all the curves that I have seen are best described using a weighted approach as opposed to straight least residuals squared and never pass through zero (unless you force them to!). From what I gather IDMS might not produce a nice linear plot...
R