How much will a yeast starter "foam"?

I've just put together a DIY stir plate using a coffee pot (and cpu fan, hard drive magnet) which is working very nicely with a test run of sugar water (very quite, nice 3" tornado).
I plan on brewing this weekend, and this will be my first yeast starter.
Mr. Malty calls for a 1.38 liter starter for my recipe. I only have about 1/2" head room in the coffee pot. I plan on covering the coffee pot with foil. How much foam does a yeast starter create? I am concerned about the starter lifting the foil off the pot due to foam (increasing chance for infection). Should I scale back the amount of wort in the starter?
Buying a flask is out of my budget this time around.

I make a lot of yeast starters with stir plates. I use one gallon jugs. I get them for a buck a piece at the apple orchard. I have had 3/4 of a gallon going on a stir plate before and it blew off. That's way more head space than you're working with there. I'd say guranteed you'll have a mess on your hands. If I were you, I'd cut the 1.38 liters in half and do it in two steps. That way your coffee pot will only be about half full each time. Settle out the yeast after the first step (put in the fridge over night if you have to) and then decant/siphon off the first round of starter beer before adding the next step. This is what I do if I need to make a bigger starter than my jug can handle.

You can use a drop or two of fermcap to prevent foam in a starter, works great. Also can depend on the type of yeast used, some yeasts don't foam at all on a stirplate, others like belgians can go crazy regardless of volume. I agree with BBL_Brewer, a smaller starter stepped up a few times will be much less likely to overflow.

I just did a small 1 qt starter in a mason jar and had it less than an inch from the top and had no issues. Any krausen that developed was sucked right back in the vortex.

I have never had a starter foam and I use a stirplate as well. I usually use the foam control drops though.

I should add that I use mostly ale strains.....occasionally a califronia lager or a kolsh. I don't have the setup to do true lagers yet. I also use plenty O2 and run my stir plate at a lower rate of speed. Just enough vortex to keep the yeast in suspension. I have an easier time identifying activity levels in the starter this way. I can tell visually when the yeast become active and when they are done and ready to settle out.

Can you explain the 2 steps a little further?

Do I add the whole package of yeast to 1/2 of my total wort needed for the first run, then refridge/decant. Then add another 1/2 of my total wort the next day?