If you are talking about a what/if scenario about Britain (and Russia) fighting Germany without the US then you also have to look at other consequences of that. The Allied bombing campaign will not be as effective without the USAAC or will Allied air superiority in the West occur as it did. It will also affect naval warfare in the Atlantic and changes in real world industrial figures and German strategies are also likely.
I am still convinced the Germany had the capacity to bomb Britain. The He-177 despite its limitations could reach industrial centres in the UK from many locations in occupied Europe, unless it was flown from eastern locations in Germany such as Berlin. Also if we are talking about a war where the US is not directly involved then we also have to look at the development of other German long ranged bombers which never got off the ground in reality due to the war ending in 1945.
Did you look at
http://www.ibiblio.org/hyperwar/USA/...hip/index.html
There is a lot of data about lend lease shipments to the Britain and the Commonwealth that needs to be examined.
Figures for merchant ship losses in the Atlantic don't completely match wherever you look. Some merchant ship losses data from WW2aSS may have been amalgamated with other theatres, and ships captured and damaged but not sunk may also have been included in loss figures.
Another decent site on line is.........
http://www.naval-history.net/WW2Camp...tlanticDev.htm
........although I'm not sure about his sources.
The link I gave you for US Lend Lease figures also has very good information about the Battle of the Atlantic
https://www.ibiblio.org/hyperwar/USN...dex.html#index
https://www.ibiblio.org/hyperwar/ETO...009/index.html
https://www.ibiblio.org/hyperwar/USN....html#contents
http://ibiblio.org/hyperwar/UN/UK/UK...dex.html#pagev
And a great many other topics as well......
http://www.ibiblio.org/hyperwar/
Another good site about U-boats is
http://uboat.net/index.html