contribution home running costs

I have been asked to take over a client who runs a small limited company from home.

His previous accountants are claiming 28% of all his property running costs such as mortgage interest, rates, light/heat etc on the basis that he uses 20% of the property living space for business purposes only, plus 10% of 36% of the remaining living area. He was given a spreadsheet to complete in order to make the calculation.

I am concerned about the capital gains implications when claiming such a large/specific amount, am I over reacting.

Is it reasonable to convince HMRC that say a study/office in a residential house is 100% business use ? 

Some feedback would be appreciated.