I never thought of trying that, however the problem I run into is I really need to know the tempo I'm going to land at. Sometimes it's in the middle of a song and I need to have everything tie together before it goes back to a certain tempo, etc.
So for most of what I do, using the percent decrease isn't terribly helpful. However, it could be very useful for rits on outros where nothing's coming back in and it doesn't really matter all that much. Thanks for the suggestion.