That'd of course depends on how particular you are with exposure. Grey card is poorman's best way to obtain ideal exposure since the camera's meter will be calibrated to 18% with it.

Here's a great article about it:
http://www.ephotozine.com/article/Guide-to...eld-light-meter

There's also illustration of pictures taken with exposure measurement under reflected light (what built in camera meter is based on) vs measurement under incident light.

I think for convenience, you'd probably just want to rely on matrix metering. Measure a frame where the light is most evenly lited, lock ev, and recompose.