If you are using the online version, click on the income tab, scroll down to employment expenses and answer the questions. The percentage usually used to determine the business portion of home expenses is the square footage of the office or business space divided by the total square footage of the residence.
http://www.cra-arc.gc.ca/tx/ndvdls/tpcs/ncm-tx/rtrn/cmpltng/ddctns/lns206-236/229/cmmssn/hm-eng.html
