>>721124851
It's at the final corner, not after final corner.
The entire Corner 4 is your possible activation timing, and if it activates on the yellow zone, that's mid-race, your ult is useless. If it activates on the purple zone, now that will be good.

It's too small of window to be good, but if you want to go gamble, that's fine. You can calculate the chance yourselves by dividing the length of last spurt on the final corner with the length of the final corner itself.