leveldifficulty
very easy1-5 (1D)
easy6-10 (2D)
moderate11-15 (3D-4D)
difficult16-20 (5D-6D)
very difficult21-30 (7D-8D)
heroic31+ (9D+)
scalemodifier
character-
speeder2D
walker4D
starfighter6D
capital ship12D
death star24D
distancedifficulty
point blank range1-5
short range6-10
medium range11-15
long range16-20
character-advantagebonus
only slight+1-5
good+6-10
decisive+11-15
overpowering+16+

Called Shots:
Making a "called shot" against a small target:

target's sizeadd difficulty
10-50 cm long+1D
1-10 cm long+4D
<1 cm long+8D