You would have to know what the districts would look like without "gerrymandering" and that's impossible to know unless you know how the districts would be designed and on what basis. I'm not defending gerrymandering, just pointing out that there is no magically "correct" way of designing a Congressional district.
The apportionment and districting processes are pretty arcane. The objective elements are pretty clear. The Census determines what the size of an average Congressional district should be (roughly 710,000 post 2010). One district is assigned to each of the 50 states. After that a formula is used to assign additional districts, which ends up determining how many each state has. The largest district is the single district serving Montana (around 900,000 -- too small to divide into two districts) and the smallest is Rhode Island (500,000 plus). Within a state, each of its districts should have roughly the same number of people. In other words, a state can't create one district with 100,000 people and another with 1.3 million people -- that's the concept of "one man, one vote" at work.
So you've got to figure out how to divide up a state into a bunch of roughly equally sized districts. But population is not evenly distributed throughout states. There are densely populated areas and sparsely populated areas. You can't simply say "this county" is a district or "this city" is a district. Geopolitical units have to be divided and/or combined, in whole or in part. Existing geopolitical boundaries cannot simply be followed.
So at that point, a subjective element is inherently injected into the process. Someone has to decide how the lines should be drawn. It sucks, but that's the way it is. (I should note that in my state, Virginia, redistricting has cost Democrats seats overall, but it also has strengthened Democrats hold on other seats. Under the pre-2010 district lines, my Democratic Congressman won by the slimmest of margins. After redistricting, he cruised to victory.
Anyway, my point isn't to defend gerrymandering. Its to suggest that the alternatives are difficult and would not necessarily guarantee a Democratic house. There are objective measures for creating districts -- ones that rely as heavily as possible on contiguity of communties and existing geopolitical boundaries. But even the application of those formulae may require choices (go north, south,east or west in looking for a contiguous area). Plus populations shift over time within those areas.
Having a non-partisan group devise congressional districts is something that needs to be done to take the process out of the hands of self-interested officials (of both parties). But even an independent group needs to guidelines for how to set district boundaries. Should they try to make each district as evenly divided among likely voters for each party (based on past registration, precinct voting results, race and other demographic features?) If they do, some areas that are pretty homogeneous are going to have to be divided quite awkwardly to create "balance".
Again, I'll say it again. I don't defend the current system and believe it needs to change. But I don't think anyone can objectively prove what the results of the past election would be without gerryamandering unless they can show what the districts would actually have been under some alternative redistricting plan (keep in mind that the 2000 districts were also impacted by some degree of gerrymandering).
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