The calculation is number of hours over 12 then number of days 5 over 7 x space used over total x total household expenses. Those percentages that accountants use only work for some businesses not one like ours. The higher some of those numbers are ex 12 over 24 vice 10 over 24 and amount of space used so the more of your house you use the better as compared to stuffing it all into a small area does get you a bigger deduction but it also comes at a cost of working longer days and having daycare mess everywhere.