My CP18 is 89% of my CP7 (but I'm a sprinter, which raises my CP7 a lot). So, using your numbers, then you might target 275W and see how you feel. You can always make it up later if you need to. Go up in 5W increments for 2-3 minutes to see how it settles in.
+1 to negative splits if the course supports it. It's much better to approach your limit from the bottom than to start beyond it and try to recover.