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Calculating the sample size for 2 sample means

Hi,
I'm trying to calcuate the sample size for 2 sample means. The difference between the means has to be less than 10%. What do I input into the 'difference to detect' box? Is it 0.1 or 10? Is there another more appropriate way to do this? The power has to be 80% or higher and I have historical data to calculate standard deviations.