The simple, accurate way to monitor asthma The patented dual colour sliders on the asmaPLAN+ define the management colour zones set by the healthcare professional for the asthmatic to follow. 100% is the asthmatic's best. The yellow zone will typically be set to start at 80% of the asthmatic's best while the red zone will typically start at 60%. Variability of PEFR of greater than 20% should alert the asthmatic to a possible asthma attack.