Where should I have got this rate from? I have used the standard gravity acceleration, rounded to two digits after the decimal point, as it is used commonly... at least in Germany it is used like this

But the value does not vary too much.
But... I forgot something

I said 1,51 (or 1.51; I mean the number between 1 and 2), but actually now I got 19.62. I tested it - using 2 digits for the gravity acceleration rate and the most exact values for the other numbers, you come quite close to the correct number.