If Trump gets 50% of the remaining delegates and 25% of the "other" delegates (make the rules so that Rubio delegates who became unbound when Rubio dropped out stay unbound), that's about all he needs. If he can't get those totals, there is something wrong.
Rubio hasn’t technically dropped out so they are still bound to him. Apparently, his disdain for Trump is great enough that he is being careful to not surrender those delegates, yet.
It is an interesting election year.