Unlike child support, spousal support is often very difficult (if not impossible) to modify after the divorce is final, so getting it right the first time is a must!
My ex made $22K and I made $135K. 90% of divorces settle and never go to trial. He wanted the equity from the house so badly ($180K) that he just took the money and ran. I never have to pay a cent in SS. BTW He is also a bipolar drug addict who tried to kill himself and us with carbon monoxide poisoning while on drugs.
My lawyer said that the worst case for me would have been 15% of my gross for life. More likely would have been $1000 a month for about 6 1/2 years (we were married 19 years). Each state has their own set of rules regarding fault, etc. In IL, fault does not come into play re: SS. (Although it was a huge factor for custody.)