I think this is a fair assessment of the situation:
http://www.al.com/news/index.ssf/2016/03/presidential_primary_schedule.html
Trump currently has 678 delegates, 559 short of the number needed for the nomination. His closest rival, Texas Sen. Ted Cruz, has 413 and Ohio Gov. John Kasich has 143. About 60 percent of all Republican delegates have been awarded so far, with Trump winning 47 percent of those. If that pace continues, Trump could end up about 100 delegates short of the 1,237 needed at the convention. To win the nomination outright, Trump needs to win more than 60 percent of the remaining delegates.
The previous pace of delegate percentage is a proper measure because of the remaining 19 states, 11 are winner take all, 3 are winner take most, 1 is direct election, and 4 are proportional. Prior to March 15, most were proportional and winner take most.
This was demonstrated on March 15, when with mostly winner-take-all delegates, Trump came away with about 250 of 400 or 63% that day. Cruz failed to take a single state. Kasich won’t win another state probably, and Trump is leading the polls in most of the remaining states, and especially in the large ones.