I've been asking this question and can't really get a straight answer. I just play with the yeast calculator until I get some sort of logic that works in my head and then I go for it.
I'm thinking you may be able to go straight to a 3L starter. Put it this way: with an average gravity beer (1.050-1.060) and a yeast packet with "average age" (let's say a month old), you generally need to almost double the amount of yeast. Usually this means making a 1-2L starter. Your stats sound about the same, so I'd say you need to make a 3-4L starter with the yeast you have. Forget the step up.
Here's a thought process that might help: It sounds like your yeast is slightly older (a month or more?) so the calculator is spitting out 300mL for each batch. Remember, though, if you make a starter, you get to reset the viability back to 90-100%. So check to see how many mL you need of yeast if it were freshest. That's how much should be on the bottom of your flask after you make the starter.