This Vimcast (http://vimcasts.org/episodes/converting-markdown-to-structur...) recording is an example of a quite complex macro for converting (a specific file's) markdown to HTML. At the beginning of the video you see that they save the macro to the "a" register. You can record macros of similar complexity to each of the other letters of the alphabet, to get some idea of the maximum complexity (though I tend to stick to about 3 or less in a single session).