I've experienced few external gauges that actually read correctly. When we're on the dyno, I compare the readings and find that the owners haven't calibrated them correctly. Our techs can tell by ear if an engine is fat or lean. I've heard all kinds of expert opinion and even had them work on our dyno. I've found that sometimes the recommended fuel ratios aren't what an engine wants. It takes a lot of experience and testing to know wha you can and do and what to avoid.