I think it is simpler than that. By Q3 2015 they are going to be running probably around 1500 a week run rate. Compare that with Q2 of this year at 800 a week, that is an 87% jump in Revenue. With Profit margins continuing to go up, and the MX helping to push the ASP up higher, lets say 30% at 110k ASP with something crazy like 16,000 vehicles produced... that is 1.76Bn in Revenue for Q3/2015 and 528M in Gross Profit. That is a LOT of income, and at some point in 2015 they will not be expanding at any faster of a rate of deployment so SG&A and CapEx costs for SuperChargers and Stores and Service Centers will flatten out (as opposed to getting a 20% jump each Quarter like we are seeing now).
Model 3 R&D is likely to taper off for a bit, somewhere in there, because they will be well finished with the engineering prototype and won't move forward with the serious R&D costs there until 2016 (just a guess, but the MS and MX have had similar curves). All the MX R&D should be well finished by then...
I really think it is just a simple combination of massive increase in deliveries and a massive flattening out of spending. Which we really NEED to see at some point if they are going to be able to start self-funding not just this Gigafactory but future factories (cars and batteries)
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http://seekingalpha.com/article/2504255-tesla-elon-musk-unveils-secret-2015-margin-target?uprof=45
So add to that line of thinking, here is someone else's number crunching suggesting that they only need to hit around 1.5Bn in revenues with 15% operating margin, and allows for Tesla to spend 225M in CapEx and they will break even. I feel these are some really low balled numbers, but it gives you another path to cash flow positive in 2015, again, without needing to take into account really anything other than MX and MS car sales.