The gain is the sale proceeds minus the purchase cost, and it cannot go below zero on this page. Tax is the gain times the rate you enter. The field starts at 15%. Net profit is the gain minus that tax. The rate is not chosen from US brackets or from Canada’s inclusion rate.
A purchase at $20,000 and a sale at $40,000 is a $20,000 gain. At 15% the tax is $3,000 and the amount left is $17,000. At a 0% rate the tax is zero. Enter Canada’s inclusion math yourself if you want that result. This page will not apply it.
It does not apply Canada’s capital-gains inclusion rate. It does not apply US long-term brackets, the net investment income tax, or a wash sale. Losses on this form do not create a negative gain.