taxable benefits

Can someone remind me how taxable benefits are calculated. If you received say 400.00 as a taxable benefit does this get taken off your tax free allowance or does 20% of 400 (80.00) get taken off your tax free allowance.
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However personal allowances ( i.e.tax free earning amounts) would have to be restricted by the £400 in order to collect the sum of £80.00 in tax if the tax rate is 20%.