stepping up a starter

I am brewing a Munich Helles this weekend and I only was able to get 1 Wyeast pack of Munich lager. According to a pitching rate calculator, I would need a 4L starter on the stir plate based on the package date of the yeast. I only have a 2L flask. I want to step it up once, but I don't know how much volume for each step. Perhaps it is just a 2L starter into another 2L starter after it ferments out?

That should work.

Is there some type of calculator out there that takes yeast viability and step sizes into play?

The wyeast calculator handles stepped starters a little easier than the mrmalty calc. If you want to figure out multiple steps, though, you might have to do it by hand using a growth to inoculation rate table.