How to create a Cumulative Total formula from varied data

[Addict]

I'm trying to make a cumulative total formula for a dial guage i have set up to measure displacement. The gauge only goes up to 50 so it has to be reset when it approaches that value, and sometimes it doesn't work properly and is reset earlier, or just reported as not working. I would like the cumulative total to equal the cumulative total of the last reading if no reading data is added and the instrument to be reset multiple times. The formula is for the cumulative total column. The data I have set up in the sheet as follows:

Date	 Reading	Working? Reset?	Cumulative Total
25/05/2012	2			0
26/05/2012	4			2
27/05/2012	14			12
28/05/2012	27			25
29/05/2012	25			23
30/05/2012	37			35
31/05/2012	40			38
1/06/2012		No		38
2/06/2012		No		38
3/06/2012		broken		38
4/06/2012	0		reset	38
5/06/2012	2			40
6/06/2012	6			44
7/06/2012	23			61
8/06/2012		No		61
9/06/2012	23			61
10/06/2012	35			73
11/06/2012	5		YES	73
12/06/2012	3			71
13/06/2012	4			72
14/06/2012	4			72
15/06/2012		Nope		72

Any help would be tremendously appreciated!