One must measure according to the customs of the country and not change from that at all. In a place where they are accustomed to use heaping measures, he may not level off [the top] even by the desire of the buyer in order to pay him less. And in a place where they are accustomed to use level measures, do not heap up [above the top of the measuring cup / spoon / etc…] even by the desire of the buyer who would pay him more. Because the Torah is concerned about false measures, lest a mishap will be caused by it. [E.g.] that a viewer may see that they measure accordingly and he would imagine that this is the way of measuring in this city, measure that way for another person who also doesn’t know the actual custom, and mislead him.
This is more on the subject of false measures, and how the prohibition even goes beyond using them to cause theft.
To again repeat the Qitzur Shulchan Arukh 62:1:
This is the first question that they ask a person at the time they bring him to justice [after his passing], “Did you conduct your buying and selling with trustworthiness?
Not just honesty,emunah.