When reporting business use of home expenses do you report before or after tax cost?

Assuming you're filing self employment taxes and not filing GST/HST, in that case report it with HST.
If you're not filing GST/HST then report it before taxes.

Here is a link for supplemental calculations https://www.canada.ca/en/revenue-agency/services/tax/businesses/topics/sole-proprietorships-partnerships/report-business-income-expenses/completing-form-t2125/calculating-business-use-home-expenses.html

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