New fuel flow vs old

I am trying to understand the basic calculation for finding new fuel flow rate.. Written as: rate of flow=_/-- new pressure ÷ old pressure x old flow rate. Could you explain the square root part as well as how do you calculate the original flow rate ie "old flow rate"

Thanks

I think I have it... Basically the old rate is the bhp x bsfc ÷ # injectors x duty cycle? Then take new pressure divided by old pressure... Find square root and x old flow rate