David,
There is no reflective mode on the L758DR apart from the spot meter. With the dome down it is measuring incident light with a cosine response (for flat copy when parallel to the flat copy; or for measuring one light at a time when pointed at the light from the subject's position - which may not give the recommended exposure).
Onesix,
With the dome up it measures in incident mode with a cardioid response (ie heart-shaped). This is intended to measure incident light for 3-D subjects, and it is best not to place it in front of the subject, but in place of the subject. This allows light from behind (except directly behind) to influence the reading - with the dome pretty much facing the camera. This gives a suggested exposure rather than the contribution from individual lights. It needs to be used with intelligence in unusual situations.
In general, when using continuous lights or strobe, I use it with the dome down when setting up the lights to assess the contribution from each light (pointing it at the lights and shading it from other lights) and in spot mode from the camera position to check the results (if I don't simply take a digital picture). Spot mode is excellent for easily judging the overall brightness range.
In sunlight for landscapes one would use it with the dome up, pointed at the camera. One reading in full sun and one reading in the shade. This indicates the approximate brightness range of the scene (by inference) and allows you to peg the exposure where you want it.
Best,
Helen