Capital Gains Tax on Gifted Shares Help

I have some time as not due until next year but want to work out what i need to save for capital gains tax purposes. I was gifted shares in a company i work for a few years ago the share certificate at the time said 60 ordinary c shares @£0.01p each, however as a benefit in kind HMRC valued the shares at £3K meaning i paid 20% Tax on the gift paying £600.00 back in 2020. Since our company has been sold and my shares on sale was value at £23,856. however only 66% of those shares were paid ion completion so pay out was £15,904 with the remaining 33% in loan notes deffered for 3 years. I understand no capital gains on the 33% as not received this and wont be due until 3 years time when i benefit from the shares but i received 66% minus c£828 for legal fee's etc... so received cash in bank £15,076. I am looking for advice what i would need to put on the form as cost of purhase everywhere i ready is the market value at the time of the shares being gifted so am i right in thinking this would be the difference of £3K the market value set by HMRC at time shares gifted or cost of purchase zero as they were gifted shares or as i only got 66% would market value be 66% of the market value set by HMRC in 2020. The only information I have that shows this is an end of year tax return for 2019/2020 stating benefit in kind £3K and £600 tax was due.

Hi 
As HMRC valued the shares gifted to you at £3000, this would be your acquisition costs.  
As 66% of the shares were disposed of you would apportion the acquistion cost accordingly.  
The gain would be the difference between your disposal value and acquisition/ disposal costs.
Thank you.