Goodness of fit function

Have several hundred thousand records in an Access Database.
Exported select records to Excel.
There are thirteen months of values per record,
along with a derived Year-To-Date value.
Used a weighted formula provided by Management
to calculate the Year-To-Date (YTD) value for each
record from the other thirteen values. YTD is calculated
in part from the thirteen values, BUT other variables
are part of the formula also.
Is there a statistical function in Excel that displays the
probability that one specific value, (in this case Year-toDate)
was derived from the group of original (thirteen) values?
example:
values: 114 228 439 683 798 838 798 252 483 585 373 381 441
and my weighted Year-to-Date is 521.
Is there an Excel function that gives me the probability of
how well the value 521 fits into the other thirteen values?
Like chi-square maybe. Covariance. Or Correlation. I dunno.
Trying to prove to management I replicated their
formula precisely without having to laboriously re-create
their stinkin' formula in Excel with the original data.
Original data table is VERY large with many variables
affecting the Year-to-Date calculation.
For example, if the result of this Excel formula determined
there was only a .0001% chance that 521 DID NOT
"fit" (maybe not the right word), or could not be derived,
from the other thirteen values,
Management would be appeased.

I'm more of a finance person than a statistics person, but under Excel add-ins is a Statistics Package. It includes some of the following formulas:

CHIDIST
COVAR
CORREL

Don't know if they do what you want, but maybe it will give you a starting point of where to look.