European pilot here :) it's also not a European thing. It works the same here as it does in the US, you use the published number in the flight manual. But this

European pilot here :) it's also not a European thing. It works the same here as it does in the US, you use the published number in the flight manual. But this plane had significant modifications done. And if you do things that significantly change performance, you'll need to get updated performance data. Or the provider of the supplemental type certificate that allows the modification has to provide an updated flight manual with that data.