The best VMG can be found on your dashboard, where the compass rose is located.
When the wheel turns green as you spin it, it means you have found the best VMG.
More about the VMG :
The VMG (Velocity Made Good) is a compromise between heading and speed.
A helmsman who sails too close to the wind might reduce the actual distance travelled, but will move slower. Increasing the angle of your boat increases your speed, but also the distance left to cover.
The right compromise is a good combination of distance, heading and speed.