You might be using the wrong function as well that might not correspond to the formula that you're trying to get.
CREATE TABLE #Heights(
INSERT INTO #Heights
SELECT SUM(SQUARE(x.Height - y.Mean_Height))/COUNT(*) - 1, VAR(x.Height), --Variance of a sample
SUM(SQUARE(x.Height - y.Mean_Height))/COUNT(*), VARP(x.Height) -- Variance of a population
FROM #Heights x
CROSS JOIN( SELECT AVG(Height) FROM #Heights)y(Mean_Height)
DROP TABLE #Heights
EDIT: And I just now noticed that you're working on SSRS and not T-SQL. The solution might be similar, hopefully.
How to post data/code on a forum to get the best help: Option 1 / Option 2
Are you seriously taking the advice and code from someone from the internet without testing it? Do you at least understand it? Or can it easily kill your server?