A positive displacement Sprintex S5-335 screw type supercharger can take as much as 30 horsepower just to turn it when it's pumping hard, so if you are starting with a standard 180hp Range Rover spec 3.9 in good health, you're now down to 150hp. Of course, this before you've put any of the compressed air the blower makes back into the engine.
At this point you should keep in mind you'll need roughly 3psi of boost just to get you back to the original 180hp, at which point you would be better off with no compressor at all because you'll just be burning more fuel to turn it, and get you back to the same 180hp the naturally aspirated engine started out with.
If you don't have an intercooler or charge cooler then 6psi would be a likely safe boost ceiling, so that's 3psi to get you back to 180hp, plus whatever the remaining 3psi gives you above that. Common folklaw states you should enjoy roughly 10hp for every psi of boost, this is of course an over-simplification as it's actually a percentage increase not a fixed 10hp per psi.
None of this is easy to estimate as the percentage will change across different compressor types, diffent engines, different elevation levels above sea level, different temperatures etc etc etc. To asses the effciency of your setup you really do need to start with your engine's naturally aspirated power output before the blower was fitted, you then need to confirm exactly how much pressure over atmosphere (boist) you are forcing into the engine?
You may also find you actually only started out with 170hp, and you're getting more like 15psi per psi of boost, so at 6psi that would be 170 + 60 = 230hp, which wouldn't be far off your quoted 240hp
