I completely agree. Based on your calculations (I did not double check them) - if you think the difficulty level will increase by 30% every 11 days for the next year (which is already an unusually fast recent growth rate), you will STILL make 30% on your money, which is 3 times as good as the stock market long term average.
Yes, 30% gain if you had started a week ago, about 0% if you start in 2 days and ca. -21% if you start in two weeks. Even if you think this growth will slow down it is not very reasonable to just use some fixed difficulty level for your calculations.
Anyway, the point I was trying to get across is, that time is an issue here: no matter _how_ fast difficulty increases, the time in the beginning will be the most profitable (except you count on it decreasing again). Which brings me back to my original question: when do you think you could have your machine up and mining?
The only reason I mentioned the 25% number is that I heard a projection that by 6 months from now we would be at 25%. Even if you only got in 6 months of mining at 25% capacity (starting now) you would end up with 45 BTC at the end of 6 months. Will we have 25% capacity in 6 months? more? less? there is some risk involved in making that guess, that's the entire point. I talked about it at length.
25% mining efficiency in 6 months is pretty optimistic IMHO - that would boil down to an average of only 0.77% network growth per day, whereas we are more at 2.5% right now. But maybe I'm just too pessimistic
Anyway, it would be great if you could estimate how fast you think you can get started.