[G0HZU_JMR]

In the case of the Siglent analyser, I don't think you can use the +15dBm input TOI (of the first mixer) for your calculations unless you use really wide tone spacings, eg 20MHz spacing.

This is because the Siglent appears to have been designed with a different gain distribution to a classic spectrum analyser.

The old HP 8566 and 8568 analysers don't have any amplification until after the second mixer. This means the distortion performance on all but the narrowest spans will be dictated by the first mixer. Your Siglent analyser has a lot of amplification after the first mixer and so the input TOI of the second mixer will probably limit the overall input TOI. I think this means that the effective input TOI will be lower than +15dBm with 0dBm input attenuation. The datasheet suggests that it will be about +10dBm.

The noise floor with a 10kHz RBW at 10dB attenuation is going to be about -90dBm and this is equivalent to -130dBm/Hz.

Your SFDR calculation should be done at the chosen RBW setting.

I tried watching the latest eevblog teardown in your link and put together a spreadsheet for the Siglent based on the parts used in the signal path. It did show that the input TOI is affected by the second mixer.

I can remember watching the initial teardown of this analyser several years ago and I recall trying to work out the frequency plan for that analyser.