Now that the 2010 pay table has been released you can begin planning for your TSP withholding percentage. If you plan to distribute your withholding out equally over the year you'll need to have the government hold out $1,375 each month. The math then is pretty simple. Just divide $1,375 by your basic pay to determine the percentage of your base pay to allot to the TSP each month to max it out.
I've taken the 2010 pay table from Military.com and done the math for you in excel. Here's the link to the results. You'll see that a 14 year E-7 needs to set aside 35.6% of their base pay to max out the full $16,500 in 2010. A 14 year O-4 defers 20.4%.
If those numbers sound high, remember that these are pre-tax dollars, and exclude your BAH, bonuses, or other special pay allowances you receive. You really can max out your TSP.