How to calculate the capital gains with cryptocurrencies, to be correctly taxed in France?

I have been investing recently in cryptocurrencies, and if I were to make any profit, the capital gains will have to be taxed. If I have losses, I must also declare them properly. I am located in France, and the law is not clear at all (to me) (reference https://bofip.impots.gouv.fr/bofip/11968-PGP.html/identifiant%3DBOI-RPPM-PVBMC-30-20-20190902 ?).

What is clear is that if I convert a cryptocurrency into fiat (i.e. euros), the transaction could be taxed (if the capital gain is positive). What is not clear at all to me, is how to compute the capital gain.

Let’s take a simplified but realistic example.

• Say that I invest 10 euros in bitcoin on day 1.
• On day 2, I invest 25 euros in ethereum.
• On day 3 I convert 10% of my bitcoins to ethereum.
• On day 4 I convert 100 euros to polkadot.
• On day 5 I convert 25% of my polkadots to bitcoin.
• On day 6 I convert all my bitcoins to ethereum.
• On day 7 I convert 25% of my ethereum to euros.

This last transaction is the only one that could be taxed, depending on the amount of euros I obtained.

In total I invested 10 + 25 + 100 = 135 euros. Let’s say that I got back 1700 euros in the last transaction. What is the capital gain?

The descriptions of simplified examples on the Internet did not help much, because they do not take into account that it isn’t easy to "trace back", i.e. they all consider that the investor invests in a cryptocurrency that he eventually sells back to fiat, without making any transactions between cryptocurrencies, which, to me, greatly complicate things. It doesn’t even make sense to apply their formula, without specifying how to determine the price (which will be subjective, i.e. I can come up with several different ways and get a different number). In some sources (for example websites offering to perform these calculations, but at a monetary cost), it is mentioned that any transaction, even between different cryptocurrencies, must be declared.

I tried to look for open sourced softwares making such calculations, but I could not find any, all the ones I have found are closed-source and not free of charge (so we cannot even check what they are doing).

Hence the title’s question.