Gradual Tempo Change: Based on Quarter Notes?

Are the baked-in percentages of gradual tempo changes (allargando, smorzando, etc) based on quarter note=x only? I have a piece in eighths, and the allargando takes off like a frightened horse.

I think gradual tempo changes do simply apply a percentage change to the current tempo, but probably all of them are a little exaggerated at the moment.