CGT on unlisted RSUs

Hey, My company have granted me RSU which vest annually (25% each year). The company has not IPOed hence there is no quoted price. The only exit path as of today is to sell these shares back to the company at their buyback offer price. How do I calculate CGT on RSUs for an unquoted entity (cannot find the price on the vesting date) as I plan to sell shares that have vested annually in 2021, 2022, 2023 etc.?

Hi,
You can see guidance here:
ERSM20500 - Employment-related securities and options: money’s worth charge
Thank you.