you should ensure that you get 15% increment in (annual total package inclusive bonus), not just in monthly base salary.

eg

current package
basic = 3000
bonus = 6 months
annual package = (12 + 6) x 3000 = 54000


your situation + 15% base
basic = 3450
bonus = 2 months
annual package = (12 + 2) x 3450 = 48300 (you actually taking a pay cut in annual salary)


my recommendation (+15% annual package)
= 54000 x 1.15 = 62100

you should ask them for 62100 annual salary (inclusive bonus) regardless of the no of month of bonus they give.