From the table it can be seen that the pressure at 8000' is 10.92 psi

Now we've been told because of the crack in the cabin window we can maintain a pressure differential of 5psi. Therefore the air outside can only be 5psi less than that inside the cabin. ie 10.92 - 5 = 5.92

Again from the table the altitude where we get closest to 5.92 is approximately 23000' (FL230)