Complex RSSI dBm calculations with shield
All gains and losses are ratios. In dB this is done by addition, it's easier for mental arithmetic and the numbers are easier to read, but you can also work in raw ratios.

You should use the FSPL formula to find the fraction of a transmitted signal that's received by another antenna.
You can then consider the shielding to be an additional loss (ratio), and multiply the unshielded case by \$10^{-5}\$, or in dB, subtract 50 dB from the unshielded case.

The correct calculation is to use the free space path loss formula:
\$ P_r = P_t \frac{G_1G_2\lambda^2}{(4\pi r)^2}\$
(my rule of thumb: \$3\lambda \approx 30 dB\$ )

In your example, 1 GHz, 0 dBi, 0 dbi, 55 cm, expressed in dB:, \$P_r = P_t - 27.2 dB\$

Now add the 50 dB shield, for a total \$P_r = P_t - 77.2 dB\$

If you're transmitting +30 dBm, then \$P_r = -47.2 dBm\$

On shielding:
The faulty logic still in your question is to calculate loss over distance, as though the signal got attenuated as it went along. It doesn't - it just spreads out in a lossless way.  

I don't know if there's such a thing as an exact 50 dB shield. In practice the shielding effectiveness of a box is an average, changing fast with frequency, and the radiation that does leak out of the box will come out only in some directions - effectively a highly directional antenna. The figure you get in practice could vary between (say) 45 dB and 60 dB.