<p>There's a few ways of doing this, one is to take a gray card (a piece of paper or cloth that is 18% grey) and meter the light in your scene falling on it. Another is to use an incident meter, which may be a function of your spot meter.<br>
The concept of an incident meter is to measure the light coming towards the light meter instead of the light reflected off of (in this case the completely black ) background.<br>
Making a grey card isn't too difficult in photoshop. The insides of a 35mm carton of kodak film usually works too. </p>