back button focusing on the 50d

tried the back button focusing for the first time today on the 50d
selected c.fn no 3 i.e ae lock/metering +af start
pictures are not bad in terms of hitting the focus but i found it difficult keeping the small af-on button depressed : is it possible to use the * button for bbf on the 50d since it is significantly bigger and easier to press

You should be able to swap the functions of those two buttons in your custom functions.