Using the rules as they stand it is difficult to come up with meaningful challenge ratings.
Both on the ground and in space, a single lucky hit from nearly anything can kill nearly anyone.
To illustrate, last night a group of my players (levels 10-12) got ambushed by a 4th level punk with a homemade .22 pistol. (d6, 20*2)
A single critical didn't quite kill one of the player characters. 11 lifeblood in a single shot, and suddenly the communication officers intestines exploded with delight.
Under 3.0, or 3.5 ed DnD it would be unimagineable that a Challenge 3 critter would even dent a group of 10th level characters.
I've taken to just playing it by ear, the players get some experience (Gm Fiat basically) and the threat is modified on the fly if need be. For example, after nearly scragging a PC, the punks then ran away, because one of the PC's had a gun.