Wheel width vs tire width

Finally got myself a set of winter wheels. Ended up with M68s (staggered - i guess not optimal for winter, but hey.. i need something!)

So im wondering, i did some math, 8.5" in the back is equivalent to 215.9 mm. and 7.5 is 190.5, so how do these values correlate with the width of a tire? Is there a rule of thumb when it comes to tire size and wheel width?
Narrow is better for winter, but how narrow can I go? Is there a benifit (or downside) of getting a staggered winter setup, something like 205 in front and 225 in back?

Thanks guys,

Tire manufacturers have a recommended range of wheel widths that each tire size should be mounted on. It's primarity a factor of matching the widths (e.g. 205-235 is a general range for a 7.5) but it will also vary due to aspect ratio (a taller sidewall allows more flex and thus mountingon a slightly wider range of wheels).

Your 205/50/17 and 225/45/17 choices on the 68s should work fine.