Combine standard deviations

Here's what I get:

Row\Col	A	B	C	D	E	F	G	H	I	J	K	L	M	N	O	P	Q	R	S
1	n(1)	u(1)	s(1)		n(2)	u(2)	s(2)		n(1+2)	u(1+2)	s(1+2)		n(3)	u(3)	s(3)		n(1+2+3)	u(1+2+3)	s(1+2+3)
2	438	62.00	13.00		427	65.60	14.40		865	63.78	13.82		140	66.00	14.20		1,005	64.09	13.89
3	193	67.00	14.60		76	66.70	14.60		269	66.92	14.57		127	65.00	13.00		396	66.30	14.10
4	29	55.80	11.90		586	60.90	13.60		615	60.66	13.56		1,434	64.00	12.00		2,049	63.00	12.58
5	391	71.00	14.00		1,095	67.00	2.00		1,486	68.05	7.58		592	62.00	13.20		2,078	66.33	9.91
6	209	61.80	13.30		378	61.75	11.45		587	61.77	12.13		73	63.70	14.40		660	61.98	12.40
7	1,054	63.00	13.00		2,288	61.40	12.12		3,342	61.90	12.42		241	61.50	12.60		3,583	61.88	12.44
8	127	55.60	11.30		145	66.40	10.50		272	61.36	12.13		420	63.30	13.60		692	62.54	13.07
9	241	59.60	13.70		218	60.80	13.20		459	60.17	13.46		139	58.00	11.03		598	59.67	12.96
10	50	54.50	11.60		258	62.00	11.00		308	60.78	11.42		57	63.10	13.40		365	61.14	11.76
11	202	63.00	13.00		44	59.00	16.00		246	62.28	13.64		821	62.90	12.80		1,067	62.76	12.99
12	28	62.00	12.00		263	64.00	12.40		291	63.81	12.36		76	66.60	11.40		367	64.39	12.20

I2 and down: =A2+E2

J2 and down: =(A2*B2 + E2*F2) / I2

K2 and down: =SQRT(((A2-1)*C2^2 + (E2-1)*G2^2 + A2*E2/I2 * (B2-F2)^2) / (I2-1))

Then copy to Q2:S2