My job as the DM is to remain impartial and unbiased. I don't want anything. I have no preference for whether the NPC has +7 or +17.
I only interpret the rules provided by the game. The rules are supposed say whether an expert smith has a +7, or some other number.
While you might want to remain impartial, apparently in 5E the "rules" that are "supposed to say whether an expert smith has a +7, or some other number" don't exist. This was intentionally done to allow the DM to decide what that number should be.
As a general guideline, I like to think of the modifiers as representing a standard deviation or the "next level of ability" for each +1. For example, in academics I might rate:
Intelligence:
10-11 = IQ 100
12-13 (+1) = IQ 115
14-15 (+2) = IQ 130
16-17 (+3) = IQ 145 ("Genius" level)
18-19 (+4) = IQ 160
20 (+5) = IQ 175+
Proficiency:
+0 = Middle school or less (what you know or can do is based solely on your ability score)
+2 = High school
+3 = Associate's degree (2-years in college)
+4 = Bachelor's degree (4-years)
+5 = Master's degree (6-years)
+6 = Doctorate degree (8+ years)
I am sure others might scale this differently, but it is really all subjective.
So, if I combine the general knowledge about a subject along with increased reasoning, understanding, recall speed, etc. from a high IQ (say 150) then I would have an INT 16. Add to that a decade of studying "higher" levels of the topic, such as a Master's or PHD, and the total skill modifier might be +3 and + 5 for a +8 total. If you want someone to be truly exceptional, grant that NPC the Expertise feature, for a +13 total.
Think about what +13 means... A DC 20 "Hard" task would be routine. You could not even have to make the roll (assume 10) unless there is the potential for horrible consequences if you fail. And it means 1 out of 5 times you are likely to complete a DC 30 "Nearly Impossible" task (17 or higher or 20% chance).
It is said with about 10,000 hours of study/practice/use comes "mastery". I would normally rate a master's proficiency at +4 to +6, and add Expertise if I want to REALLY make the NPC exceptional, with a very good ability mod of +3 or higher, hopefully putting the NPC in the +10 to +15 range.