do I need another downspout?

Had news eavestroughs put up awhile ago, and the installer seems to have done a bad job. In a heavy rain they overflow at the ends opposite the downspout, and there's standing water in them after the rain. I'm getting the contractors to come back and rehang them, but in the meantime I wonder if I need an additional downspout. The roof of my house, in side profile, is shaped like this:
(back) (front) /\ / \ / \ ____________/ \__ |------------ <-eavestrough | 27' | |<-downspout
The problem eavestroughs run along the long, flat, back portion of the house, one on each side. I assume that by rehanging them at a correct slope I'll solve most of the problem.
But in trying to figure this all out, I've also realized that I have about 70 feet of gutter (two 27-foot sides plus a 17-foot back) all draining into a single downspout at one corner of the house.
I'm thinking of asking them to put another downspout on the opposite back corner of the house. In the archives for this group, I see that one downspout per 40 feet is standard, but that it also depends on things like roof pitch and precipitation. I'm in Toronto, so there's a lot of precipitation, and it's a flat roof (except for the small steeply-pitched section you can see above, which drains onto the flat roof and then off to the sides).
The reason I'm asking the group is that these contractors have already made a hash of the job three times (initial installation plus two half-hearted repair attempts), and we're now demanding pretty strongly that they do the job right, but of course no longer really trust them to tell us what the right way is. If it's clear that an additional downspout should have been installed we'll demand it. If it seems optional or isn't clearly necessary, I might back off on that demand and settle for a correct slope.
Thanks for any help.
Al
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He didnt check the level and install with downslope to downspouts, and that is the sign of a really crappy job. The number of downspouts needed is related more to the area of roof drained, per gutter run. One always assumes the pitch of the gutter is enough to get a good flow towards the downpipe, and the downpipe diameter should reflect the roof area drained, and the liklihood of torrential rains in your area. If there is a bow in the middle of the gutter, and nothing can be done to fix this, then your only alternative is to have a downpipe at each end.

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On 26 Nov 2003 11:23:16 -0800, snipped-for-privacy@yahoo.com (Al Sponsor) wrote:

Another downspout can't hurt, especially if there is a good sloped area to drain to and the appearance of it is not highly noticeable. Another option is to replace the exisiting downspout with a larger downspout.
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snipped-for-privacy@yahoo.com (Al Sponsor) wrote in message

There are charts that help design gutter and leader sizes. Geographic area gives rate of rainfall Roof Area then leads to sizing data. Architectural Graphic Standards & SMACNA manuals include these tables.
Tom Baker Charleston SC
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