I don't see any reason for the construction of the "Array2", at least not for anything I see in this screenshot. Maybe you need that array later, but I doubt it.
Augment simply puts the vectors side by side so you get a matrix with exactly the number of rows the vectors have and in your case five columns. cause you augment five single column vectors.
There is no reason you should define the range variable N. Its created automatically implicitely by the sum symbol. So if you don't need them later you can delete the range N and the matrix Array2. Simply replace rows(Array2) by rows(tX) (or better: spend a variable for the number of rows rather than calculating it five times), which should be the same, if you like the calculation using the whole vectors.
The difference to the version I posted is that you use the full vectors while mine uses just those elements, where the corresponding tX values are in the range you had specified (for whatever reason). So I am summing up less elements and thats the reason for any differences in the results.
In the example I am using the elelements from index 3427 (tX=39.997) up to the last index 4112 (tX=44.997, the last value, even if you would accept t-values up to 65), that is 686 elements, while you use alle 4112 elements in your calculation.
If I set i1:=1 in my sheet I get Cd=-1.11 and Cm=5.451 which should be the results you get with your formulas.
The two expressions using match calculate the indices i1 and i2 of the values in tX which are closest to tX_start and tX_end you had specified.
In the data set you selected originally tX_end (65) is larger than the biggest value in tX, so we have to be happy with 45 instead of 65.