LTD calculations with deduction being one DBA minus another DBA

We are trying to implement a new process where the ER will cover 50% of our LTD up to $4,000 at a monthly rate of .0024.  I've got that DBA set up as a benefit, no problem.   The EE can opt to get 60% coverage up to $16,666.66 at a rate of .0040 Minus the ER portion being paid.

Option A:  Monthly income to a max of $4,000 X 0024 =monthly cost  = ER cost

Option B:  Monthly income to a max of $16,666.66 X .00410 = monthly cost

LTD Option B (shared cost) -LTD Option A =  XXX X12 month/ 26 pays = EE Cost per pay.

How do I set up the EE cost per pay.  I can do Option A and Option B but don't see how to deduct A from B to get the EE's deduction.