The markets have Trump winning the GOP nomination at 60% and the likelihood of a contested convention at 57%, or a 43% chance the nomination is decided on the first ballot. It's inconceivable that Cruz wins in the first round. Consequently, we're looking at an 17% chance accorded to Trump if it goes into extra innings. Even that strikes me as far too optimistic. After the first round, there will be mass defections of Trump's initial pledged delegates. Party rules only require fidelity on the first ballot. Forget about poaching delegates from Rubio and Kasich, Trump won't be able to hold onto his own.
Trump has to win in round one. That does not necessarily mean that he needs to hit 1,237 in pledged delegates going into the convention, however. There are 109 unpledged delegates in the initial round who are not formally tied to any particular candidate. My best guess is that Trump will come in with 1,200 pledged delegates. The strategy then becomes getting ~37 of the 109 unpledged delegates to vote for Trump in the first round.
Can Trump get one-in-three of these unpledged delegates? Nine of them are from Guam, which is at the southern end of the Northern Mariana Islands where Trump won 73% of the vote. Another 9 come from the south Pacific as well, in the form of American Samoa. There are 3 in the Virgin Islands and one he'll get out of North Dakota. The balance in play are from Pennsylvania. They are the biggest question marks. Many have said they'll back whoever wins the popular vote, presumably even if it's just a plurality, but these commitments aren't backed by anything. If Trump gets close to 50% in the state, persuading these people becomes realistic.
Paul Manafort should be working exclusively on the unpledged delegates in the first round. Trying to win in subsequent rounds will be as futile as king Canute trying to control the tides.