Measurement described here uses a spot meter/narrow angle internal meter and an 18% reflective gray card  (rather than an incidence meter).  The ratios are exponential of the base of 2 (think binary) at the point of the subject, perpendicular to the lighting.  The ratios are often achieved with setting the distance from the subject.

1 2 4 is pretty much the standard portrait setting.  1 is the key light, the brightest.  2 is the fill and should read one f/stop dimmer.  4, the shadow fill, should be three f/stops dimmer.  In a standard range, 6 is solid black shadow, ambience too low to show detail.  This is reserved for dramatic effects and is generally shunned for most purposes.

1 3 5 is 2 and 4 f/stops difference in light reflected from the gray card.  There is less contrast, and it's a bit darker in the shadows than a 1 2 4.  1 2 5 will deliver a primary contrast as a 1 2 4, but the shadows will be deeper.

Many studios only worry about key to fill and express ratios 2:1 or 3:1, and let the shadow levels find their own with the bounce of the room as ambient.  But for changing environments, it's better to specify the shadow fill.

Google "studio lighting ratios" if you want more info, though most of it doesn't adhere to a zone system, since most use incidence meters in their examples.