so for them you have to sell your coins everyday at current price, for them you must not hold anything, and if you decide to hold then is a gamble, if the coin your mined is at $1200 and then it rises to $2000 and then you sell then by your example you will have to pay the tax based on the $1200, $800 is profit, now if the price crashes to $120 like now then you will have to pay by your own pocket $1080, so if tax = 10% then based on the $1200, you will have to sell your eth and pay the $120 as tax with it hehe.

I'm glad, i live in a country where I only pay tax when i sell something. If I buy 1 billion dollar of eth then they will tax me only when i sell it and they will charge me 15%. If I dont sell then they will never tax me.