I think Neva Parker said a 1L starter will give you something like 20% growth and a 2L starter might give you up to 100%. And, yes, I use a stir plate....If you're making a larger starter, does that mean intentional overpitching?
Not that I really fret over a few 10s of billions of cells here or there, but I usually try to follow George Fix's recommendation of 0.75e6/mL/degree Plato. Depending on freshness and fill level, a pack would usually end up being about half Fix's recommendation for what most brewers would consider a typical beer. For a 12-13 plato beer, a 2 liter starter should be just about right, assuming 100e9 cells in a pack, about 90% viability in a reasonably fresh pack and a single doubling of the yeast population on a stir plate.
For an older pack, a 1 liter starter might bring you back up to the original cell number in the pack, or, if you get lucky, you might hit Fix's recommendation if you got a really fresh pack filled on the high side, starting with 130e9 cells, have good air transfer into the flask (loose cover, large headspace), and got 1.5-fold increase in cell number.
What I meant to be asking, though, is if you intentionally underpitched specifically for Weizens.