The way that Stars Without Number currently works a character must make a successful Navigation/Intelligence roll on 2d6 against a base difficulty of 7. Navigators can attempt to “trim the course” by increasing the difficulty by +2 in exchange for increasing the effective spike drive rating of the ship by +1, but a navigator cannot trim a course by more levels than their Navigation rating. Travelling within a solar system requires Navigation-0 skill, but doesn't require a skill check unless some special hazard is involved or the navigator attempts to trim the course (difficulty 6, +1 for each level of course trimming). This skill check difficulty is increased even more based on the age of navigational charts that the Navigator uses to chart their course, and if they are charting a course into unknown territory the difficulty jumps by +4.
That's the rules as written.
Rather than fiddle with a bunch of difficulty ratings and modifiers, a simpler approach would be:
If a pilot is flying under the spike drive parameters (e.g. flying 1 hex with a drive-2 engine) and not attempting to trim the course then there is no need to roll. Trimming a course means shortening the time in hyperspace by one day. Default is 5 days (my game, it's 6 days by the rulebook).
Navigation+Intelligence, base difficulty 7, difficulty increases by +1 for every level of course trimming.
With no limits placed on the amount of course trimming a player might attempt.
Succeed at the roll and you arrive safely.
Fail the roll, and the player must select two.
Fail the roll by a margin of 4+, the player must select three:
- drive down, ship cannot be operated until repaired
- computers down, piloting at -1ongoing until repaired
- navigation systems down, courses cannot be plotted until repaired
- off course, GM randomly determines current location
- travel time is twice as long
- life support systems down, crew will die if it is not repaired immediately