To overclock a GPU you simply use a utility like MSI afterburner and there will be the voltage for your GPU. Your voltages on your motherboard or in BIOS are not what you want to adjust. Also every GPU is different so there is not really a voltage that we can tell you, it depends on the individual GPU, everyones is different. Also you don't really calculate that. You simply open a utility like MSI afterburner and start by increasing the core clock in small increments such as 5-10 mhz, and then running a benchmark (like unigine heaven or 3dmark11) or a game to see if it is stable. If it is then you continue to increase the GPU clock by the same increments until either:
a) Your game or benchmark crahes
b) You see artifacts or tearing
c) your temperature is over the safe limit
If its case a or b you bump up the voltage by two increments or so and run the benchmark or game again. If it runs well then you are stable at your current clocks. If your stable and your temperature is not too high then continue increasing you clocks, until the same thing happens. Then you move on to the memory, and the process is similar. However don't increase your voltage past say 1.25v. Also watch your temps carefully, make sure you do not burn your GPU as I am not responsible for any damage.
Also what power supply are you using, you have to make sure it is enough.
Hope this helps!