Using your example and Donald's formula (which is the correct way to calculate this):

TO = 30 seconds

LO = 5 inches

LN = 8 inches

LN/LO = 1.6

(LN/LO)^2 = 2.56

TN = 30 seconds *2.56 = 76.8 seconds

With some papers and large jumps in exposure length, you may need to add more time for reciprocity failure. Or you can also compensate with aperture if you have some room to breathe there.

Lee