Mistress, that's a great way to look at it and approach a session, especially with someone you're meeting the first time.
In reading the responses here I think "service tops" are more like A + B = C (or C, D, F, G etc. depending how many fetishes/activities you're trying to cram into a session).
A Mistress who asks the right questions and a sub who outlines what he or she likes, especially before a first or second session, should be unscripted, ad lib. With two experienced players the best sessions are like two jazz musicians who agree to jam a popular song. You know how you'll start and probably know how you'll finish, but the trip in between should be filled with some pleasant surprises for both. And an audience, if you have one.
I've posted the following observation before on other sites and kinda got in trouble for it from Mistresses. (jbeast, in trouble for posting something? No way! LOL!) In my experience, the best Mistresses or dominant women I've served, all have some type of artistic or performing arts backgrounds. This is NOT to say the Mistresses who are plumbers, accountants or administrative assistants are average. But I've served women who were or are dancers (Broadway types or erotic), actors, photographers, artists and writers. They use their imaginations, and forces you to use yours, usually for great experiences.