[cr0sh]

Thinking a little outside of the box - but if you could sort the lines by amount in ascending order, then total from both ends, take half after each addition, then shift a pointer to where "about half" would be based on where you were in the totaling (and which end) as you go.

I don't have any specific implementation - or if this idea would even work - but I do think sorting the lines first might be of help toward a better solution (I could be completely wrong on this). As well as tackling the totaling and checking from both ends of the list.

You'd have to keep track of everything, of course, in order to know at the end where in the -real list- (unsorted) the half-way total point is at. If this idea has any merit at least.