Your best approach is to assess rent levels in your local area for offices of a similar size. You need to take into account all the usuals - amenities, parking, etc, just as if your home-office was provided by a third-party. Get your $/sq ft and work out the monthly amount.

With this figure, you need to then work out what % of it you can charge. If the space is used exclusively for the business, charge 100%. If it's used about half the time, charge 50%, etc.

I would strongly advise you to do two things - 1. make sure your accountant and your attorney help you get this squared away. 2. document everything about how you arrived at the cost. Nothing fancy, but dates, realtors, addresses, $/sq foot. A simple table will do. By doing these two things, if the IRS should come around to chat, you should be covered.