That's why the best advice is to follow the advice in the book.
The POH in my PA28 says absolutely nothing about short field take off. All it says is that the normal T/O procedure is, no flap, full power and rotate at 74 mph. This is the configuration for the take off performance data in the POH.
However, on grass, I was trained to use 2 stages of flap , rotate at 65 mph and accelerate in ground effect to 74. Many years ago I was taught, in extreme cases, to increase flap to full at 60 mph and then gradually reduce back to 2 stages while accelerating in ground effect. I was also taught this, incidentally, as a rough water technique in a Cub float plane. I have never done it without an instructor in either case.
Can anyone tell me what effect the use of 2 stages of flap is likely to have on take off distance. I still use the normal calculation in the book because its the only one I've got.