This side view shows the cheapie drinking cups I use. I put about 1/2 inch of pure perlite in the bottom, and then the seedling mix. They are 9 ounce cups, about 3 1/2 inches tall, and a bit less than 3 inches across the opening at the top. I really like being able to see if just a bit of water is left (condensate on inside of cup, below soil level). You can see such condensate if you have a very coarse seedling mix.

Some folks don't like to put anything at the bottom of the pot, they use seedling mix all the way to the bottom. Having tried it this year I will keep doing it, it seems beneficial.