As far ad I know, lowest RPM with turbo fully active. With diesels its around 2200 rpm, but you need to find yourself. Try to search for kW vs RPM charts, o;-)r torque vs RPM. You can usually tell where the boost is active.
And get either OBD2 adaptor and Torque app, or ScanGauge or UltraGauge or similar device. Ot will tell you more.
Oh, I almost forgot: Welcome to the forum!
