The Gunson-Gastester product im sure is good, but it is hard to beat the AEM wideband air fuel ratio gauge. they are compact and 12V powered so you can have it mounted in your car for real time AFR measurements, for those without a dyno.

AEM wideband

One good rule of thumb with webers is "if in doubt with the choice of the calibrated parts, choose the smaller one" Many a time have I seen webers with chokes way to big that gas speed are near non existent and idle and main jets that are too big for the application.