SPI Temperature sensor interface with Arduino Uno
delay (0.425);// wait for conversion 


You can't delay a fraction of a millisecond. You can use delayMicroseconds though:

delayMicroseconds(425);


However, you don't need to sleep at all. 


  The conversion clock for the part is internally generated so no
  external clock is required except when reading from and writing
  to the serial port. In normal mode, an internal clock oscillator
  runs the automatic conversion sequence. A conversion is initiated
  every 400 ms. At this time, the part wakes up and performs
  a temperature conversion. This temperature conversion typically
  takes 25 ms, at which time the part automatically shuts down.
  The result of the most recent temperature conversion is available
  in the serial output register at any time.


You have your chip select pin backwards:


  To ensure that the serial port is reset properly
  after power-up, the CE must be at a logic low before the
  first serial port access. The serial clock is active only when CE is
  high. For correct data synchronization, it is important that the
  CE be low when the serial port is not being accessed


So it should start LOW, and be set HIGH before you read, and back to LOW afterwards.

You need to read 2 bytes, not one, then combine them in the right way to make a proper 8-bit value:

digitalWrite(A0, HIGH);
uint8_t b1 = SPI.transfer(0x00);
uint8_t b2 = SPI.transfer(0x00);
digitalWrite(A0, LOW);

int8_t = (b1 &lt;&lt; 1) | (b2 &gt;&gt; 7);


The reason for that is that the first bit of the first byte is a "leading zero" so you need to discard it, then add the first bit of the second byte as the last bit of your value, discarding the remainder of the second byte, since you specified only an 8-bit resolution in your question.