I like china white's method:

Take a look at front end volatility. For the S&P (a reasonable enough proxy for the Dow) I believe this is derived from the VIX or VXO. At present it is about 18.5% (annualised) as they are both around 18.5 (not absolutely certain about my interpretation here).

Obtain the DAILY volatility given 252 trading days in the year:

18.5/square root(252) = 1.17%

Dow is 10250. 1.17% is 120 pts a day.
SPX is 1118. 1.17% is 13 pts a day.

This is the daily range one can expect.

If you pocket half of these points daily you are doing very well and a third is not bad. So I guess that means 200-300 Dow pts a week is the target to meet...