I looked into this some more. I modified the code so that the Enter key also selects the text so that you can type over it without having to do anything with your mouse. That will be 3.1.
I should mention that the shipping app has a behavior that is probably useful to you. You can use the up and down arrow keys to manipulate the value. For example, you type 1.0 for exposure and hit Enter. You can then use the up and down arrows to bump the number some (and option-arrow and shift-arrow will manipulate the values by smaller or larger amounts).