Example -
Annual premium: 1200.00
Monthly premium: 120.00
Policy lapsed in 01/05/2010
Annual Interest: 8% from 02/05/2010 to 06/05/2011 (16 months)
Monthly premiums needed to reinstate: 17 (2040.00) = (16 months x 120)
Reinstatement Interest: ?

Formula -
Reinstatement Interest = (120*1.08^(16/12) - 120) + (120*1.08^(15/12) -120) + ..+ (120*1.08^(1/12) - 120)


Same way you can calculate your query interest.

P.S. In actual scenario there are not two options.