Its not accurate at all. I would go by airflow if anything to gauge how much power you are making. The rough estimate is 10hp for every 1lb/min of air. That is a better gauge of how much power you are making....its not dead on by any means, but its much better then the HP calculation routine.

There is a way to make that more accurate, but you would have to search through the Link archives on the board to figure that out.