Sekonic L 508 ?
All light meters are calibrated to 18% gray. So your meter in your DSLR should measure light the same way as the Sekonic meter. I use Sekonic 608 and every time I need to be accurate with my light measurement, I will compare my readings between the camera meter and the Sekonic. Almost always, they agree. I use the Sekonic mostly to measure the ambient light as I find this as pretty accurate.
All meters are not calibrated to 18% gray. Every light meter will be factory calibrated differently depending on the maker, and mode (incident vs reflected). Generally the incident meters (or incident mode if dual type) will be close to 18%, while reflected (or reflected mode if dual type) will be calibrated closer to a 12%-14% gray. This applies to both Sekonic and Minolta meters.
Camera TTL meters are calibrated along the reflected meter 12%-14% gray method. A simple check is to compare reflected meter (orTTL) readings using a known accurate gray card against the incident meter. When reading the gray card with a TTL camera meter, it is important to set the lens on infinity and fill the frame completely with the gray card image. A gray card made by Douglas Photographic that is both an accurate 18% reflectance (.74 density) and neutral in color (equal amounts of R/G/B reflected) would work well. I have tested numerous gray cards and few are truly neutral and accurate 18% gray. Sekonic also offers a couple of calibration targets which feature calibrated patches on one side and a true 18% gray on the opposite side, but these are a bit more money than the Douglas card I mentioned.
Unless the reflected meter was recalibrated, it will be close to a half-stop off from the incident meter. While I do not recall if the L-508 allowed individual mode calibrations, most of the models that replaced it allow you to clibrate the spot mode independently of the incident mode. This means you can tweak the meter to give identical readings to an 18% regardless of whether you are in spot mode or incident mode. This is very important if you use a metering system the way I do.
Mention has been made in this thread about the importance of 1/10 stop readings. Since most cameras are only adjustable in 1/2 or 1/3 stop settings on the lens or shutter speed, the 1/10 stop readings do not contribute much. However, if you are shooting in a studio using some of the pro flash units that can be adjusted in 1/10 stop increments, this is a welcome feature.
I hope you and the OP find this info useful...