(Yn+2 - Yn-2) / (Tn+2 - Tn-2) = X
Where n is a data point.
So if I have a data table like this:
Y | T
-------
0 | 12
1 | 13
2 | 14
3 | 15
4 | 16
5 | 17
For Y = 2, the calculation would be:
(4 - 0) / (16 - 12) = X = 1
The formula obviously won't work for the first and last two data points, but that's fine.
