Calculating capital gains tax

Hi all,

Every time I've sold some bitcoin, in order to work out the capital gains tax, I'm supposed to know how much I paid for that bitcoin. The problem is, say I sell 1 bitcoin on an exchange. What if I purchased parts of that bitcoin at various different times over several tax years at several different prices. And what if some of the bitcoin originally came to the exchange from my hardware wallet having been sat there for several years. In other words its really complicated! And I have lots of bitcoin sales like this.

My tax accountant suggests calculating the tax just by looking at how much money I sent to the exchange from my bank versus how much profit I sent back to my bank. It certainly simplifies things but I'm worried I may end up underpaying my tax.

I played around with coin trackers tax tool but it didn't seem to be giving accurate results(I may have not used it correctly, not sure)

How are other people handling their capital gains tax?

Many thanks.