Anyone know how to add remaining time on the LCD display of i3?

Anyone know how to add remaining time on the LCD display of i3? I found the forum posts which talks about adding % complete (1st post linked below). I am looking to replace the elapsed time to remaining time using formula (elapsed time)/(% complete) *100. I understand it is a very rough estimate, and appreciate if someone can help with the exact formula for Marlin.

http://community.robo3d.com/index.php?threads/full-graphics-lcd-controller-percent-complete-mod.3835/