Grading system for extentions of boulderproblems and routes

Has there ever been made a calculation method or rule of thumb for the grade of routes and boulders that are put together by two existing routes/boulderproblems with known grades? For instance how hard does a sequence of moves on a sit start have to be to make an 8A (stand) become 8A+? What is 7A + 7A? Does the same rule apply for both routes and boulders?