grm, I'm not sure if there is any formula, and case of your 8" cable shows it. What counts into calculation of transmission line effects is transition time that can vary between 5ns for dedicated transports to over 50ns for average gear. In addition the highest slew rate is usually at the beginning of transition (first knee). When calculating trace length on PCB it is a little easier, because logic family is known, PCB material is known, etc.
At the end, result is what counts and your ears are the best judge.
At the end, result is what counts and your ears are the best judge.