The 23% is not CGT by the way, it is an exit tax. My understanding is after year 8, you pay 23% on any gain. Then after year 16% you pay 23% on any gain etc. If for example, you want to encash your ETF after 10 years, you calculate the tax liability as normal (i.e. 23% of the total gain) and you get credit for the tax you have already paid.
So for example, after year 8 you pay €1,000 in tax. You encash your ETF after 10 years; the value of your holding increased by €10,000 over the ten years so €2,300 is due in tax. You have already paid €1,000 however, so only an additional €1,300 is due.
I coulld be wrong though.