Excel 2007 : Interpolation in excel

I have a (I believe) text issue within a function.

I am using the =Spline(Xarray, Yarray, GivenX).

This is an engineering extension pack that follows the same function rules as all others.

So in column A is array X, column 2 is array Y...Given x is solved so no issues.

The number of rows with data in column's 1 and 2 are the same...but unknown!

I need to search these columns and input data from a1:a(whatever), same for B.

So I need the function to read something like

=Spline(A$1:A(count(a1:a1000), etc...). Know what I mean? Thanks!

From Help:

OFFSET

Returns a reference to a range that is a specified number of rows and columns from a cell or range of cells. The reference that is returned can be a single cell or a range of cells. You can specify the number of rows and the number of columns to be returned.

Syntax
OFFSET(reference,rows,cols,height,width)

Reference is the reference from which you want to base the offset. Reference must refer to a cell or range of adjacent cells; otherwise, OFFSET returns the #VALUE! error value.

Rows is the number of rows, up or down, that you want the upper-left cell to refer to. Using 5 as the rows argument specifies that the upper-left cell in the reference is five rows below reference. Rows can be positive (which means below the starting reference) or negative (which means above the starting reference).

Cols is the number of columns, to the left or right, that you want the upper-left cell of the result to refer to. Using 5 as the cols argument specifies that the upper-left cell in the reference is five columns to the right of reference. Cols can be positive (which means to the right of the starting reference) or negative (which means to the left of the starting reference).

Height is the height, in number of rows, that you want the returned reference to be. Height must be a positive number.

Width is the width, in number of columns, that you want the returned reference to be. Width must be a positive number.