Best Practices for creating a formula dealing with SELECT

Hi, 

We are trying to create a formula that will subtract the sum of all the List Items, (excluding List Item 'Other'), from the overall Total of all the List Item totals added together. We were thinking of using the SELECT function to pull in the amounts for each List Items and then summing those all together in a SYS Module and then pulling in the result to our main Module. Please advise how this can be done with Anaplan best practices. 

We would want the Above Formula to be subtracted from a sum of all the other List Item Amounts. 

Thank you