Here is a random Junkrat ult. https://youtu.be/mYYab3XFa3E?t=42s
Notice how the 0% pops up before he even starts moving it?
Which is exactly how it should be. Otherwise Blizzard would basically be encouraging people to not take out a ultimate when it was nice and easy. There would be no point if Junkrat would just have his tire shoot and could just move back and try it again.