Good voltage and resistor value for string of LEDs
Since the relation between voltage and power is quadratic, the obvious answer is "as little as possible".

Allowing for 3% variance in VF gives us just under 13V, so that is a reasonable starting point. Solving for R gives 22ohms, a E24 value. This will result in a 8.8mW loss, which even a 0201 resistor can handle.