It’s great to climb on ropes with obvious middle marks, but someday you’ll definitely be climbing without one.
When you’re in this situation, and have multiple rappels, put a temporary middle mark on your rope with a strip of tape or chalk.
It won’t last through the day, but it’s going to make setting up all your subsequent rappels faster and easier.
Some tape works fine.
Give your rope a dip . . .
and have an instant chalk middle mark to quickly set up all your rappels.