~In the passout seat, the common practice is to use a range of 15-18 HCP.
~Purists require that the 1NT hand hold no more than one doubleton, and some players go so far as to require at least a half-stopper (Qx) in the doubleton. The more conservative requirements are definitely in the minority.
Opening 1NT with a 5-card major
~Some experts recommend (insist?) that you always open 1NT with a 5-card major, a semi-balanced hand and 15-17 HCP. Other experts are just as adamant that you should never open 1NT with a 5-card major. In the examples that accompany such articles, the recommended bid works every time and the other option either won’t work or makes the bidding more difficult.
Obviously both camps cannot always be right. It would seem that there should be some additional guidance to help players decide which recommendation to follow. As you might expect, there is a third camp of experts that suggest that you never open 1NT with five spades and that you only open 1NT with five hearts when you also hold three spades (in case partner transfers you to hearts.
You might also choose to open 1NT with a 5-card major if in 3rd seat after partner has passed.