Make a bigger starter the first time and you won't have to keep stepping up.
You can also make more cells with less volume on a stir plate than without.
For a 5 gallon batch, I've never needed more than a 1.5L starter to get into the ballpark of my inoculation rate except with a few lagers and a barleywine And those were all 2L starters.
Adding additional wort to an already propagated starter is ineffective.
Think of it this way, you start with 100b cells and make a 1.5L starter and allow it to go for 24 hours. Let's say you get 250b cells at the end of that.
Now if you just add an additional .5L of wort, you're effectively doing the same thing as pitching 250b cells into a .5L starter, so you'll get virtually no growth.
If you want to increase the propagation rate after the initial 1.5L starter, you should do the following:
Cold crash your 250b cells
Decant off the starter wort
split them into 2 separate 1.5 L starters and put those each on stir plates.
Your lower pitch rate into these two separate starters will ensure higher growth rates. So instead of getting 350b cells in a single fresh 1.5L starter, you'll get 250b in EACH of your two new starters.
That being said, there's no reason to need that many cells for a 5 gallon batch. Unless you're making a ridiculously big beer, the ballpark of 220b will get the job done cleanly and efficiently.
Hope that helps and isn't too confusing!
