Hi,
We cannot commment on your calculations or provide financial advice. Income tax liability is calculated before capital gains liability is calculated. The more of the basic rate band that the income uses, the less of the lower rate of capital gains is available, increasing the amount taxed at the higher rate of capital gains. You will need to add up all of your income, from employments, self employments, interest, dividends and so on, to work out how much is taxed at 20%. Any of the basic rate band unused, can be applies as the lower rate of capital gains.
You can report and pay the capital gains online using the link below or by completing a Self Assessment Tax Return.
Report and pay your Capital Gains Tax
Thank you.