This page is not finished yet, since i found a easier solution to the problem.
Send and measure the time of 1Byte and 3Byte.
The 1B time is the offset, the difference with it and the 3B is the tSymbol.
divide the length of the (payload + 3.5) by 5.5, multiply by tSymbol
and add the offset to get a length, that is pretty close.