Here's my understanding, and my reading agrees with this method.

Let's say you have a camera and you are pointing your camera at a gray card at some distance.
1) Set your meter at reflective mode. Stand at where your camera is, and point your meter to the gray card.
2) Set your meter at incident mode. Stand at where your gray card is, and point your meter to the camera.

You should get the same reading. When you do this, be VERY careful your own shadow does not obscure the sensor on the meter. I am also assuming you have a SPOT meter with just enough coverage to JUST read your gray card. If it reads more, your bets are off. You'll have to get closer to the card.