If i were the man that calculates this, i would calculate the computer-error range. Then human-error range. Then think of the worst-case(maximum error). Then multiply that error-fraction with total fuel. Then add that much fuel onto the existing fuel.

Errors would come from:

 - Measurement(both human and devices)
 - Calculations(both)
 - Probability of fuel leak.
 - Probability of sudden changes in atmosphere.
 - probability of finding free funds(credits or money whatever it is)

Lets say worst case is %0.5, i put %0.5 more.

Lets assume all computers and all the silicon-germanium were stolen by aliens. What do we do without digital devices? We make it hand-calcuated. Lets assume this error is %40. Then we put an extra fuel tank to be sure.