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.

It won’t last through the day, but it’s going to make setting up all your subsequent rappels faster and easier.

It's not good practice to try to permanently mark 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.

(Note: at one point I recommended dipping the middle of your rope in your chalk bag as another way to give it a temporary mark. However, I have since read some Edelrid technical documentation that excessive chalk on a rope may be harmful, so I don't recommend that anymore. Here's that link if you want to check it out, see page 34, ”Dirt”.)

If your rope doesn’t have a middle mark and you want to add one permanently, a good choice would be the Beal rope marker. It has a specially formulated ink in a handy dispenser that's designed for use on climbing ropes. It can be messy. Do it outside, lay down in a plastic bag under where you're working, and wear gloves.