The easiest thing to do is rate something from 1-10 since you know what a one for you is and ten for you. You can give the same answers as somebody else with this scale, but are the answers the same, or did you select the exact numbers but you meant something else.
In the Netherlands, where I live, a seven or an eight is good. You do not get a nine or a 10. We consider that too much, not because we dislike it, but because we score very conservatively. This mindset is disastrous for anybody trying to do anything around Net Promotor scores.
The workaround is that the people asking for an NPS tell you that if you are happy or satisfied that you should select “the very satisfied” option, which is at the end of the scale. And even while you might not be delighted, you might do it because the instruction tells you you should choose the extreme option over de moderate ones. This instruction helps people get some promoter scores (9s and 10s). However, they might not capture the real intends from people.
Net promoter score (NPS) is a widely used market research metric that typically takes the form of a single survey question asking respondents to rate the likelihood that they would recommend a company, product, or service to a friend or colleague. [..]The NPS assumes a subdivision of respondents into “promoters” who provide ratings of 9 or 10, “passives” who provide ratings of 7 or 8, and “detractors” who provide ratings of 6 or lower.
NPS Wikipedia page
To not fall into this trap, you could introduce good decision hygiene. Decision hygiene consists of ensuring accuracy, breaking the tasks /judgements down, making them independent, and refraining from adding any information that could create bias.
First of all, make sure that not any of the scores is an individual expression (personal interpretation of a judgement) but an accurate one.
Break it down and make it independent
Please do not make it one single question/score. Make it several independent tasks. Break down complex judgement (are you likely to tell a friend about it, how difficult is it to work with us, are you happy with the service delivered) into smaller fact-based assessments and ensure that each one is evaluated independently for others.
No extra information when not needed
Do not add information that people do not need to complete a questionnaire or survey, not even when accurate.
If you want to get feedback from customers on what they think, make sure that they can express themselves in a way that does justice to their viewpoints.