Gutbucket, what you're trying to do is ambitious but from a quick look it seems to me that you're using the dB correctly. (I'll have more time later to look carefully at what you posted.)
But my point is that to reach a meaningful, practical result with this type of calculation--one with real predictive value as to what human ears are likely to hear in playback--you would need more detailed information than what you've got. You'd need to consider different noise spectra (frequency distributions) as well as the time-domain aspects of noise (e.g. the relative contributions of impulse or "shot" noise to a signal as compared with steady "background" noise). Those aspects of noise aren't just incidental; they can make truly major differences in the audible result.
Also, and I don't like to say something that could be taken as disparaging a serious microphone manufacturer, but--DPA is currently alone among the "majors" in the way they specify the noise levels of their products. The only numbers which they generally give in public follow a standard method (A-weighted "rms") which gives the least practical information of the available standard methods for specifying microphone noise. It is the one which gives the lowest number in dB, however, so naturally it's the one used by marketing people for "specification battles."
Compare this with any spec sheet from AKG, Beyer, Neumann, Sennheiser or Schoeps and you'll also see a second figure which is normally some 8 - 12 dB higher (CCIR weighted "quasi-peak"). That figure has far more predictive value in terms of the audible result of a microphone's noise, and is currently the one to pay attention to if you simply must have one single number.
When I get time I'll post some frequency graphs to illustrate what I mean. When you plug a condenser microphone into a preamp, you get an overlay of the noise from both of them. With most high-quality equipment the microphone's noise will dominate in the low frequencies while the preamp's noise will dominate in the upper midrange and above. From the dB figures published in spec sheets, or from a "shorted input, maximum gain" measurement of a preamp, you'd never know this but it's what almost always happens.
The "crossover" point depends greatly on the preamp gain you're using, and that in turn depends on what you're recording and how sensitive your microphones are. But a preamp's noise performance at one gain level simply can't be extrapolated from knowing what it is at some other level--different circuits react differently to changes in gain.
So this has to be measured under more realistic conditions, and can't be predicted from spec sheet values alone. If you do that, you'll get a number and your arithmetic may check out but the result may be very, very wrong. You might well choose preamp "A" over preamp "B" on this basis when preamp "B" would actually be several dB quieter under real-world conditions.
I can tell you for example that in my own tests, one preamp which has a reputation for not being super-quiet (the FMR "Real Nice Preamp"--it's small and not very expensive, and its manufacturer almost apologizes for its A-weighted full gain EIN spec) turned out to be among the very quietest when it was driven by the source impedance of an actual microphone, and when its gain was set to what I actually use for music recording.
--best regards