Need help level shifting
Consider this circuit: 







simulate this circuit &ndash; Schematic created using CircuitLab

To calculate the values, the TL431 (ref input shorted to anode) needs 1mA to work, so let's
set the current ignoring R3 to about 1.5mA. So R4 ~= 5K, use 4.7K. 

The resistor ratio R1/(R2||R3) = 0.5 for a gain of 1.50 so let's arbitrarily set R1 = 10.0K

We know for an input voltage of 0V the output voltage should be 1.25V. 

So 1.25V/10K = 2.495V/R3 

So R3 = 19.96K, 20.0K is close enough  

We also know R2||R3 = 20K, so R2 ~= open. 

Go back and check our R4 assumption is okay- worse case is with Vin = +0.5V so
the current through R3 is about 3V/20K = 150uA leaving plenty of margin for D1 to
work. 

Although you could use the power supply as a reference, usually that's a bad idea because it is less accurate and more noisy than even the cheapest reference (like the TL431). If you want to use the power supply, just re-calculate replacing your negative power supply voltage for the reference voltage. 

Note that my example with the TL431 happens to work out to eliminate R2 as a happy coincidence. The implication is that Vref must be &lt;= -2.5V, so you cannot use a reference of (say) -1.25V.