Very difficult to calculate as I withdrew some and invested more as well.
You must also realise that most investors cash in at some stage of their lives to perhaps buy a home or a pension or whatever and be subject to CGT.
If you held your investment for 10 years and sold now, you would pay 33.3% CGT from your base cost 10 years ago.
It does not make much diffs if you sell after 5 years and reinvest again for another 5 years and sell again as your base cost will be from 5 years ago and not from 10.
If you have an investment of a few million, you might actually end up paying less tax overall by selling and reinvesting as I did due to the fact that our CGT is lower than most countries and WILL increase.
I am by no means advising selling. The only reason I sold was if Greece had defaulted and became the catalyst for the demise of the Euro, I would have made a massive loss. I wrote off my expense as insurance fees albeit a small fee.