Thanks for your responses!
Ok,  yesterday I did a quick test.  Using Canon’s meter I exposed a gray card for normal exposure, so Zone V (not shooting RAW). I switched to ML spot meter and the reading was 49.  I did the same comparison up and down the scale and got the following:
1. (-3) Zone II = 11%
2. (-2) Zone III = 20%
3. (-1) Zone IV = 34%
4. (0) Zone V = 49%
5. (+1) Zone VI = 69%
6.(+2) Zone VII = 83%
7. (+3) Zone VIII = 96%
Then I re-read the article by Gisle Hannemyr (ftp://http://dpanswers.com/content/tech_zonesystem.php) and noticed that he has the percentages that correspond to the various Zones and my numbers come very close to his.  I think this shows that the proper reading for a gray card (18% reflectance), in order to place it on Zone V, should be 49/50%, the middle of the scale, and not 18%. I think the reason for this is that a camera meter is measuring tonalities, not actual reflectance.  When I think about it, if a meter is going to measure the percentage of reflectance an object has, it would have to know the strength of the light source.  So I think that the scale 0% to 100% simply represents the tonal range from black to white (much like 0-255), not the actual reflectance of an object, and we expose any given object in a way to place it where we want it on that scale.

https://www.magiclantern.fm/forum/index.php?topic=15830.0