calculating claims expense

Transposed for convenience:

	A	B	C	D
1	Month	Prem	Claims
2	Jan 2013	$ 100.00	$ 4.17	C2: =SUM(INDEX(B:B, MAX(ROW($B$2), ROW()-11)):B2) * 50% / 12
3	Feb 2013	$ 110.00	$ 8.75
4	Mar 2013	$ 120.00	$ 13.75
5	Apr 2013	$ 130.00	$ 19.17
6	May 2013	$ 140.00	$ 25.00
7	Jun 2013	$ 150.00	$ 31.25
8	Jul 2013	$ 150.00	$ 37.50
9	Aug 2013	$ 140.00	$ 43.33
10	Sep 2013	$ 130.00	$ 48.75
11	Oct 2013	$ 120.00	$ 53.75
12	Nov 2013	$ 110.00	$ 58.33
13	Dec 2013	$ 100.00	$ 62.50
14	Jan 2014	$ 100.00	$ 62.50
15	Feb 2014	$ 150.00	$ 64.17
16	Mar 2014	$ 160.00	$ 65.83
17	Apr 2014	$ 110.00	$ 65.00
18	May 2014	$ 90.00	$ 62.92
19	Jun 2014	$ 140.00	$ 62.50
20	Jul 2014	$ 100.00	$ 60.42
21	Aug 2014	$ 110.00	$ 59.17
22	Sep 2014	$ 130.00	$ 59.17
23	Oct 2014	$ 160.00	$ 60.83
24	Nov 2014	$ 120.00	$ 61.25
25	Dec 2014	$ 90.00	$ 60.83

	A	B	C	D	E	F	G	H
1	Month	Jan 2013	Feb 2013	Mar 2013	Apr 2013	May 2013	Jun 2013	Jul 2013
2	Prem	$100.00	$110.00	$120.00	$130.00	$140.00	$150.00	$150.00
3	Claims	$4.17	$8.75	$13.75	$19.17	$25.00	$31.25	$37.50

B3 and copied across:

=SUM(INDEX(2:2, MAX(COLUMN($B$2), COLUMN()-11)):B2) * 50% / 12