In the 400m you want to get out near 100% the first 50m. Get into a nice easy relaxed state. Relax your upper and keep your foot speed. You should come through 200m at +.50-2.0 of your 200m PR. The 3rd 100m and most important you want to begin a gradual accel through the turn to alleviate any decelaration because of the curve then you bring it home with everything you have.
