Directors rent for use of facilities at home

I am happy enough with the background to this much debated subject. I have a London based client who wishes to charge her company rent for the use of a spare room as an office. I would like the client to make the calculation giving due consideration to apportionment etc etc. We will declare the rent on her SA return giving relief for the relevant expenses thereby not creating a personal tax charge for her.

Is there any specific HMRC guidance on what can be included here? Am I right in thinking that the principles in the BIM which relate to self employed are not neccessarily appropriate for the director making the calculation?

I appreciate that 99% of our small limited companies will be just as well to claim the £3 per week use of home. In this situation I think the amount involved will be substantially higher and I want to ensure my client is given the most appropriate advice in this situation.