I'm not familiar with the SVI amplifier. Perhaps you have a link to the spec's?
I think you're estimate of loss is about right. However, decibels are added, not multiplied. If you deliver -72.4 dBm to the antenna and it has 3 dB gain, you should expect -69.4 dBm at the antenna terminals. If you deliver -69.4 dBm to the input of a 15 dB gain amplifier, -54.4 dBm would be the expected output. After traveling through coax with 2 dB loss, the power to the tuner input would be -56.4 dBm.
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