If, instead of stamp duties, we'd had an annual property tax, like a grown-up country, the fall would have been nothing like 60% (though it would still have been more than 7.4%).
Where does the 7.4% come from.
In most places property tax rate is set of what budget is required from it.
If a city needs €1bn from property tax, has 1m residents with an average home value of €100,000 then you set property tax a 1%.
If there is a cash, the average property value drops to €50,000 but you still need €1bn you set the rate to 2%.
You basically start with what you need and figure out how to get it as opposed to setting tax rates and seeing what happens.
Earnings can change quickly but you house value will be based on a known previous date.