LTD Deduction changes

Our Co. is looking to changing the way that we calculate the Long Term Disability DBA's for the employee paid portion. Currently, we have a company paid LTD for up to 50% of our wages and the EE can buy up a policy to 60%. They are going to raise the monthly income limit and the cost to the EE will be a calculation minus what the company pays for the ER portion.
Ex: LTD Option A: Monthly income up to a max of 4,000 X.0024= monthly cost = ER cost
LTD Option B: Monthly income to max of $16,666.66 X .0040= Monthly shared cost
the EE deduction would be Option A minus Option B X 12 /# of pay periods
I can do the Option A and Option B DBA's but how do I get the deduction to be based on the Calculation of Option B minus Option A?