Curlyb

My wife bought a property in 1999 with her spouse for £65k and lived in it as main residence for 30 months. Thereafter property was rented out. Spouse died in 2009. Value of property at that point is estimated to be £165k. Property was then fully owned by my wife and rented out until being put up for sale in Dec 2024 for £316k. My understanding is that the purchase price for cgt purposes is (65+165)/2 = £115k. Is this correct? If so, how and/or where is this declared on cgt submission?