I have to say that this appraoch doesn't sound right to me. A goal of 12% or so a year is achievable in the long run as an average, but to try to actually make that much each year is faulty.
The market will have up years and down so to try to make 12% during a year where the S&P is down 10% or more (without being able to short), or to limit yourself to 12% (or to turn conservative once 12% is reached) when the S&P is up 25% or more makes this a tough proposition. But to average 12% is a different story. I do see you say "at least" 12%, which makes sense.
Good traders know (and I'm not one of them) that you cut losses quickly and let the winners run, so you should not get too conservative once 12% is achieved during a potential big year, and you can't get too aggressive in an attempt to make 12% in a down year. You'll probably be burned.
The 0.25% per week is even more problematic as there will be +1% or +2% weeks, and -1% or -2% weeks. Trying to make "just" 0.25% during the big positive weeks, and trying to make 0.25% in down weeks magnifies my concern mentioned above.
If I have this wrong, you'll have to explain it to me. Thanks!