Gary''s Student wrote...
The limitation is due to the function and not the range of points. *For
example:

=AVERAGE(A1:A256)

will work just fine. *However:

=AVERAGE(A1,A2,A3,A4,A5,A6,A7,A8,A9,A10,A11,A12,A 13,A14,A15,A16,A17,A18,A19,A20,
A21,A22,A23,A24,A25,A26,A27,A28,A29,A30,A31,A32,A 33,A34,A35,A36,A37,A38,A39,A40,
A41,A42,A43,A44,A45,A46,A47,A48,A49,A50,A51,A52,A 53,A54,A55,A56,A57,A58,A59,A60,
A61,A62,A63,A64,A65,A66,A67,A68,A69,A70,A71,A72,A 73,A74,A75,A76,A77,A78,A79,A80,
...
A251,A252,A253,A254,A255,A256)

will not work.
...

Me, I prefer to believe OPs know what they're talking about rather
than that they're so stupid they can't distinguish SYNTAX ERRORS from
calculation errors.

Excel 2003 and prior won't accept entry of your formula BOTH because
there are too many arguments ( 30) and too many characters ( 1024).

The OP said the formulas he was using worked when there were fewer
than 256 data points. If there were 255 data points, he couldn't have
been anything like your formula because even for A1,...,A255, there'd
be too many arguments and too many characters.

OTOH, if the OP were using Excel 2007 (or 2008), then your formula
*WOULD* work.

So how about making an ATTEMPT to respond to the OP on the basis of
assuming the OP *DOES* understand what he's saying?