Steam Engine is extremely accurate, IF everything is exactly as it should be, which is rarely the case. Math is math, it cannot be inaccurate, variables outside of the math are what can throw it off. For coils, people often miss the coil diameter, easy to forget and can really throw things off. Leg length will also play a factor, most people don't measure exactly how long the legs are, though this should have minimal effect. The wire consistency itself can also play a factor.
Never used Steam Engine for juice, I always use eJuice Me Up.