In Washington state is there a standard formula to calculate our 12% interest rate on spousal support in arrears?

Calculating interest on spousal support or child support isn't that hard. Each month's payment is automatically reduced to a judgment if not paid by the end of the month (forget about calculating daily amounts). Each month, the arrears accrues 1% interest. Be careful, WA doesn't allow compound interest which other states allow. When a payment is made beyond the monthly amount, it reduces the overall arrearage--the principal. Thus the arrears still owing for the following month are reduced, thus reducing the interest owed for that month.

Calculating interest by month is the easiest way to do it. Just slapping 12% on doesn't give credit for overpayments. It's too hard to try to provide an example of the table layout that makes monthly method easiest to understand using Avvo's little response boxes.

There's no specific rule or case that requires one formula, I just know I have always had great success in court when I have used the monthly formula no matter which side I was representing.

You could find a lawyer who would be willing to simply show you how to do this without having to pay them a ton of money to dive into your case entirely.