Hi,
If the property was your main residence for the whole period of your ownership, there would be no capital gain.
You need the acquistion cost when you both orifinally bought the property and the value of the property when you bought out your ex partner. Add 50% of both values together to obtain your acquisition cost. This will be be used to work out your gain when deducted from the disposal value.
You can see guidance on Private Residence Relief here:
HS283 Private Residence Relief (2024)
There is also a Capital Gains Tax calculator here:
Tax when you sell property
Thank you.