I know this question has been asked before, but I haven’t found an answer that suits my needs.

Consider the following example:

As you can see, there are three swells in this string passage. The first swells to mp, and the third swells to mf, and between that, there’s a swell to an intermediate dynamic between the first two (currently designated with ?). Each swell is meant to be distinctly louder than the last.

What should I write in the space of ? to indicate a dynamic between mp and mf? Is there something that means “louder than mp but not quite mf”? Until recently I used m, but many say that’s too ambiguous.