Question re calculation of tax

Hi all
Apologies for the annoying question but would be grateful for any answers. I need to do my 2022 tax return and have thus far avoided doing it because of avoiding this calculation.
I bought ffx pre demerger and was allocated x amount of Leo shares. I never sold any ffx shares but then sold some of the Leo shares to derisk given the ffx bomb.
At the time I was allocated the Leo shares, I worked out my buy in price...and it looks like I sold the Leo shares at a profit (though I've not even got back my original capital).
how on earth do I calculate tax owing? Is it just on capital gains using my calculated buy in price and the sell price? am I also entitled to CGT discount even if I didn't hold Leo shares for a year but held ffx for multiple years?
seems outrageous that I will be paying tax on this transaction when Ive not recouped my original amount. granted I still have ffx and Leo holdings which have now become basically worthless.
thanks very much.