There seems to be a solid consensus among those who use 15-17 1NT that a jump to 2NT after a minor -suit opening is 18-19 HCP, with 2NT as 20-22. Since a two-point range is acceptable for other notrump ranges, I am genuinely puzzed as to why 18-20 for the 2NT rebid is unpopular.
Benefits include:
Plus... it's free!