I read Whitman's 4" rule and I followed it.
I used 4" diameter pots for seedlings.
I realized that it made no sense to have 2" for side root growth when it should have been 4".
Now I use 8" diameter pots to start seedlings.
I'm raising air-pot bottom disk up to 6" from top
so I'll get 4" of soil and 2" for solid pot to hold water.
If I buy more pots for seedlings I would choose 10" diameter.