Well the answer is...you don't know with any decent accuracy, really. But then again, the amount of yeast needed for a batch is also known only to a broad estimation.
The best approach is probably to plug the amount of slurry you have into the MrMalty yeast calculator
for slurry, making your best guess as to the parameters like yeast% and such. Then use that number in your starter calc either on MrMalty or YeastCalc.com. And then maybe adjust your starter amounts upward a bit, since it's generally accepted that it's better to overpitch a bit than to underpitch.
But really, without instruments it's a seat-of-the-pants type situation.