Hi and welcome to Just Answer!
Long term capital gains are taxed at reduced federal tax rate 15% plus 9% for state income tax
If you recognize short capital loss $6000 in 2011 - it will be fully used to offset your long term capital gain - so you will have net long term capital gain of $7,000 for 2011 - and your estimated tax liability would be $7,000*24%=$1680.
If you will recognize short capital loss $6000 in 2012 - long term capital gain of $13,000 for 2011 will be taxed - so your estimated tax liability would be $13,000*24%=$3120.
Assuming you will not have any capital gain in 2012 and in 2013 and - you will use $3000 in 2012 and $3000 in 2013 to offset other taxable income
- so your potential income saving in each of these years would be $3000 * 39% (30% federal and 9% state tax rate) = $1170.
So your total estimated taxes for 2011- 2013 would be $3120 - $1170 - $1170 = $780.Total tax saving if you postpone recognition of the short term capital loss $6000 to 2012 would be $1680 - $780 = $900.
That makes a perfect sense to have $6,000 loss in 2012.
Let me know if you need any clarification.