In your formula,
=INDEX(A1:A10,MATCH(9.99999999999999E+307,A1:10))
The MATCH() function returns the relative row a value in the range A1:A10 matches, or is equal to, the number 9.9999999999999E+307. However, if it finds no match, it returns the number of rows it looked through. Note that you could use any reasonable number that will always be larger than any number that will be found in the range A1:A10. So, if the range contains the series 1, 2, 3...10, you could use 11 as your "large number". Probably, a large number, such as 1000, might be perfectly suitable, but, the excelent programmer who suggested the 9.99999999 etc.E+307 chose the largest number that Excel recognizes, just to cover all bases!
The INDEX() function returns the data that is in the cell on the row returned by the MATCH() function, counting from the first row in the range.
In your case, it should return the data in the last cell in column A.
If you need to know the last row number, rather than the data in that last cell, you could use, assuming cell A3 is the first cell in your range:
=row(A3)-1+MATCH(9.99999999999999E+307,A1:10)
Let us know if you are now satisfied with the above.