Normalized standard calibration curve
I'm not sure whether calibration for a dynamic assay is any different from one that is stable. But let's think of the situation like this. What you have done is no different from what an operator would do to obtain a calibration curve with an internal standard (IS) correction (by plotting concentration against the ratio of the intensities of analyte/IS) except you have used the absolute intensity of your background signal (no analyte) instead of an internal standard for correction. This is not correct by any standard since the background is a bad choice for correction (the background is NOT stable, and has a terrible RSD due to low counts). What you should do is to plot your calibration curve without normalising it to the background.