Buying a note that is late in it's straight line amortization schedule

[Ben Bakhshi]

With straight line amortization schedules, interest payments decline over the life of the loan.

Does anyone have some tips (or formulas, or excel tables) for calculating the actual interest collected if you buy a note X years into Y term based on straight line amortization.

Buying a "mature" note, the buyer may end up collect much less interest than on paper.