Hi Gordon,
Those vials are designed to be used for Zenith as well as Weber 3bbl carbs, hence the two set of lines. The upper ones are for the Webers and the lower set is for Zeniths.
The float level is perfect when the meniscus rests on the lower line with edges against the upper one.
Naturally, the engine needs to be running when viewing the gauge and deciding what to do.
These markings are calibrated for 3.5 psi of fuel pressure so thats the place to start the whole process.