Yeah but how many people are going to understand that. And then do that for every transaction. Also it’s interesting to see what happens if you don’t actually own the crypto. Now you have a taxable gain on your visa balance... and what happens on a loss say the next month is it offset? Like if Bitcoin moons in dec and crashes in Jan there is gonna be one hell of a tax Bill

This is an interesting question. You'd need to keep track of cost basis and gain, and specify whether you want to work on a FIFO or LIFO (more likely) basis.

There are online services that will do these calculations for you at tax time, like bitcoin.tax or cryptotrader.tax