I've done enough editing of image files to know a few things.
If you are going to have % of transparency you should have a small input section from 0 - 255 as exacting levels of transparency. Most people cannot tell the difference of 1% to about 5% level of transparency, but some might want the option to have exacting levels for better control.
Don't presume that about me. I don't like to lie as much as stretching the truth so far that is snaps back on me like a rubber band that has broke from the stress.