On the face of it, sizing a finger for a customer should be a fairly simple task. However, with the introduction in recent years of the comfort fit, which is the curved interior of a ring, the process can be plagued with errors.
Ideally, the gauge used to measure a finger should be an exact replica of the ring being purchased i.e. have either a straight bore or a comfort fit bore and be the same width. There are variations to the comfort fit that can upset the apple cart in as much as the radius of the fit can be quite shallow, heavily domed or anything in between, hence the reason to use a gauge that has the same profile as the ring being bought.
Catering for every instance of profile and width would require an arsenal of gauges that would be totally impractical to have at your disposal, so a selection of narrow (4mm), medium (6mm) and wide (8mm) with both medium radius comfort and straight bore profiles should cover most eventualities.
Using a narrow gauge of about 4mm wide to measure a finger for a ring at 10mm wide will not give an accurate measurement, as often a wider ring will need to be slightly bigger for comfort and vice versa.
Under ideal circumstances it would be advisable to take measurements at different times of the day to allow for expansion and contraction of the finger due to changes in temperature, exercise, menstrual cycle or fluid retention. As you may only see your customer for a brief period at what could be any time during a working day, you are not going to know exactly how much their finger is going to alter under differing situations.
Unfortunately you have to be guided to a certain degree by the feedback from your customer as to how it feels on their finger, and your customer's definition of comfortable can vary from one to another - especially when one customer prefers a slightly looser fit and another likes a snug fit - all very confusing!
Actually watching your customer trying on the ring can give you an idea of if it is right or not - if they are having to force the ring over a knuckle that is quite large compared to the area where the ring sits, then it may be a case where the ring needs to be a compromise between actually getting it on and off and comfort when it is past the knuckle. Rings have a tendency to be easier to get on than get off because the skin bunches up around the knuckle in the 'off' direction.
In summary - choose the right gauge for the ring being purchased and encourage your customer to come back at least once more later on in the day for a second fitting (twice more would be preferable) and never take your customers word for it when they say that they have determined their ring size by using a paper ring chart from a website or by wrapping a piece of string round their finger.