Estimate it. As I clearly said, it is mostly about increasing transparency, for a greater understanding and better policy tomorrow. Turn those that hide their wealth from the tax authorities into criminals, while making compliance easy and cheap/mostly free.

Some countries now require you to pay a yearly future-tax-contribution on financial investments, which is then corrected at time of sale (eg. potential for a tax refund after selling a stock after it had a very bad year). Good or bad, I don’t know.