Home expense rate small business

I currently write off 5% of my home expenses like repairs and utilities for my small business due to one room in the house being my office. My garage is currently full of material and tool to operate the business and I think I should be able to write off more of a percentage because of this.
This is a 3 bedroom house that is about 1500 square feet so one room and garage are used completely for business. Is there a calculation to determine percentage for write off?