Sum a column to a certain value
 

I'm working on spreadsheet that has 1000's of rows and want to be able
to add up a column until it reaches, for example, 900. And then take
the next set and of rows and do the same. So that there are several
rows "grouped" that the sum of the values in the particular column is
900. As well there will be times after the first "grouping" is set the
numbers will change so the the procedure will have to redone.

Also there maybe times when the value of 900 may need to be changed to
a different #.

Below is a very simplified example of what I need....thanks, in
advance, for assistance.

A B
Widgets 100
Things 500
More things 300
Total 900
Pieces 500
Widgets 200
things 200
Total 900