I'm trying to find out a formulae regarding the gap between Takeoff
Distances by POH and real-life measures. I did the maths for wind,
temperature, pressure altitude, runway slope, weight, etc. It's quite
detailled and effective...

But...

The only calculation I don't know how to "translate" is the difference, in
feet, between the POH speed and the speed I did during the test.

For example: If my airplane took off at 1050 feet on the initial roll, at 62
knots, how many feet will I need if left the runway at 56 kts ? POH states
it requires only 699 feet at 56kts for the same factors (temp, press, etc)!

Thank you

Paul