What is the "range" for a strong 1NT opening bid?
A lot of people will knee-jerk out "15-17." Some will start a discussion of upgrades and downgrades for this or that honor collection, will speak about tenaces and length cards and the like. You might even have discussions of controls and "three and a third's" with some.
All of this is fine when opening the bidding, before anyone has said anything. But, that only gets you so far. If partner shows you an unbalanced hand, and if you have a fit, the situation radically changes, such that your analysis should also change.
I mean, if your 15-HCP hand features the KJ2 in clubs, that seems nice. If you later find out that your partner has a stiff club, however, the KJ2 looks not so useful. If he has AQxxx, however, you love the KJ2 more tha you thought.
If the auction and knowledge changes, the "range" for a 1NT opening can wildly change, therefore, when viewed as a function of how good it fits with partner's hand. From a "Losing Trick Count" perspective, the "cover card" count probably could change by as much as three cards.
Consider a normal-looking Qxx-KQx-Axx-Axxx, a 15-HCP hand. If partner has something like 5-3-3-2 pattern, your hand has five cover cards -- the two outside Aces, the spade Queen (the agreed trump suit), and both the King and Queen of hearts (a side fragment held by partner).
What if, however, partner holds 2-1-5-5 pattern, a minor two-suiter? Now, your cover card count looks more like 2, one for each Ace but nothing else. At most,m if partner has both major Aces, you might contribute a cover card for the diamond King. This might also help if the opponents defend incorrectly.
Now, the cover card count is not as important unless Responder has an unbalanced hand and we end up declaring a suit contract, but the point seems apparent. In this rough example, the number of useful cards for a minimum hand of exactly 15 HCP was somewhere between 2 and 5 covers.
Thus, as far as cover cards is concerned, a "tight range" of 15-17 HCP is not remotely tight at all.
Keep this in mind when developing bidding agreements and when analyzing a given auction. A "maximum" in terms of cover cards is probably about six cover cards (one Ace, one side King, plus two internal King-Queen combinations. A reasonable "minimum" might be a 15-count with K-Q-J opposite a stiff, Q-J opposite a doubleton, and then only two useful cover cards. I am having trouble imagining a 1-cover-card 15-HCP hand. So, the "freak extreme" hands are 2 covers or 6 covers. Hence, the normal range is probably 3-5.
If you have the freak extreme 6 covers, go crazy. If freak extreme only 2, you might pass a forcing bid. But, 3 is a minimum (regard;ess of HCP strength), 5 is a maximum (regardless of HCP strength), and 4 covers is middling, needing more analysis.
So, the range for a 1NT opening is 15-17 HCP, or 2-6 cover cards.
BTW, notice that this phenomenon is not at all unique to 1NT openings. It is just with 1NT openings (and 2NT openings) that Openers get especially lazy, feeling that they have somehow showed their tight range by the act of opening. Not so.