Assuming that the 400 mg of medicine is taken every 12 hours, the remaining dose in your system would be:
Amount after medicine is taken: Amount after the body breaks down half the dose:
400 200
600 300
700 350
750 375
775 387.5
787.5 393.75
793.75 396.875
796.875 398.4375
798.4375 399.21875
799.21875 399.60937
799.60937 399.80468
799.80468 399.90234
799.90234 399.95117
799.95117 399.97558
From these results we can conclude that the long term effective level in your system after you take each tablet is 800mg, because eventually, the results would be 799.999999999999..., which we would just round up to 800.