As I understand it, the NF is the thermal noise at the output of an amp compared with just loading the input with its terminating impedance. Now the problem I have with your calculations is that there is no reference to the actual amplitude of the signal compared to that of the thermal noise. For example your figure 40 dbmicroV is the signal,with a S/N of 28 dB, so the noise is in the micro volts and the added noise of the first stage will add to it. If you input signal was 1V with a S/N of 28 dB, then the noise would be in the 10's of millivolts and the noise of the first stage would not add to it.
????Frank