There are many different methods for laying out the string spacing on the mandolin. I like to start with one string from each course, and as with guitars, you can choose between even string centres , even string space between each string, or you can use a string spacing rule, which gives you something in between. Likewise, the space between each string in the pairs can be wide, narrow, or graduated, typically with a bit more space between the thicker strings. Anywhere between 1,5-3 mm is normal, depending on playing style.
The distance between the edge of the nut and the outmost string slot can be pretty small, but not so much that the strings fall off the edge of the frets when fretted. You have to consider this total string spacing when you lay out the rest, so plan accordingly.
As Mario says, equal string slot bottoms are important for intonation. The string action depends an playing style, generally lower action works well for high speed and a light touch, higher action may be needed for heavy "chopping" and more volume.
Compared to guitars, tolerances are very tight, so grab your Optivisors and go slow
