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.
Note - it's not good practice to try to permanently marked the middle of your rope with tape, because it's going to probably become a sticky mess, and maybe even move from the correct spot, which could be dangerous. Only use tape as a temporary marker.
Give your rope a dip . . .
and have an instant chalk middle mark to quickly set up all your rappels.