211 CMR, § 38.06
In determining the amount to be paid by the Secondary Plan on a Claim, should the Plan wish to coordinate benefits, the Secondary Plan shall calculate the benefits it would have paid on the Claim in the absence of other health care coverage and apply that calculated amount to any Allowable Expense under its Plan that is unpaid by the Primary Plan. The Secondary Plan may reduce its payments by an amount so that, when combined with the amount paid by the Primary Plan, the total benefits paid or provided by all Plans for the Claim do not exceed 100% of the total Allowable Expenses for the Claim. In addition, the Secondary Plan shall credit toward its Plan deductible the amounts it would have credited to its deductible in the absence of other health care coverage.
211 CMR, § 38.06