While finding Vth at R2, i know the voltage at a, by finding the voltage at b, the Vb-Va will be the voltage through 1 ohm right?

 

You must find the Vth voltage for a circuit without R2 = RL = 1Ω (replace this resistor with a open circuit).

 

While finding Vth at R2, i know the voltage at a, by finding the voltage at b, the Vb-Va will be the voltage through 1 ohm right?

The best way is to remove R2 and find the open circuit voltage across a-b. You also need to define the polarity of Vth since it is arbitrary.

If you find Vba = Vb-Va with R2 in the circuit, you will have identified one point on the load line curve for the Thevenin equivalent circuit. You need to find another point or find the equivalent resistance (which, in this case, is pretty easy). Those two pieces of information would let you find Vth. But it's just easier to remove R2 and find Vth directly.

Also, don't talk about voltages through a resistance. Voltages appear across components while current travels through them.