In form T2125, there are home-use expense and car expense fields. Home use expense is based on the percentage of area you have used for the business. Car expense is based on the percentage of mileage used for the business.
Please read the CRA guide: T4002 Self-employed Business, Professional, Commission, Farming, and Fishing Income 2017.
The screenshot below shows where to enter home use expense in the form T2125, the final caculation result is populated on field 9945: