So, I went here and played around:
RealClearPolitics - RCP Republican Delegate Calculator
1. Entered vote totals based on either a) latest aggregate for state if there were a steady stream of recent polls, b) latest poll if there was only one for February or c) defaulted to current national average.
2. Simulated Carson & Kasich dropping out after Super Tuesday
Very, very rough, of course, but after March 15th (i.e. after Florida & Ohio vote, not to mention Super Tuesday) the results are roughly:
Trump: 829 delegates, 30 states
Cruz: 364 delegates, 3 states (TX, AR, IA)
Rubio: 215 delegates, 0 states (yes, not even Florida)
At that point there would be 992 delegates remaining, and 50% is roughly 1200. The delegates pledged to candidates who are not the above are negligible.
So Trump would need only roughly 400 more delegates, but Cruz would need all of them and Rubio probably couldn't win.
Obviously very rough. I encourage others with better state knowledge to play with the tool.