You can use Mr Malty's calculator to determine the starter size.
Step 1) Work out how many viable yeast cells you have in your packet of dry yeast
To do this go to the
http://www.mrmalty.com/calc/calc.html then the Dry Yeast tab, enter your OG, beer style and volume, and the date for viability.
Mr Malty will then tell you how viable your yeast is (76%?) many yeast cells you need (300 billion?) and how many packets of yeast 1.6? etc
Divide the yeast cells by the packets of yeast to get the number of viable cells in your yeast packet
in my example, that's 300/1.6 = 187.5
So, you have about 188 billion cells in your 11g packet of yeast.
Step 2) Determine how big a starter you need given your desired number of cells and your starting number of cells
Now go to the the liquid tab
Enter 188% as the viability, and that will trick the calculator into starting with 188 billion cells (yes really!)
Choose your starter type, and growth ratio, etc, and Mr Malty will spit out the starter size
for my 40L of 1.040 Ale with Intermittant Shaking you need a 1.21L starter.
Good luck