By soil, do you mean from the ground/garden? Best to mix with something like coarse sand or vermiculite/perlite so it will not hold too much water, as well as some shredded coir or leaf mould or pest moss to make it lighter and fluffier. Be sure to make drainage holes in the bottom and side of the cup.
Usually, using fluffy potting mix, filling to the very top is best because the potting mix settles down when watered. If only filled to the 2nd rim as you say, you may find that the level will go down to only mid-way up the cup after watering. Be sure to pre-moisten the soil mix to just damp consistency BEFORE filling the cup -- "squeezed out sponge" or a handful holds shape when squeezed without any water dripping, then falls apart when hand is opened and clump is poked.
I usually fill a container to the top by scooping, press lightly down to about 1/3 full, then sprinkle some more in to the top, leaving it fluffy for making holes or channels and sowing.
Basil seeds shouldn't be buried, just press into the top of the mix.
But if this is a plant -- basil will ultimately need at least 8" wide and deep container, 12" wide and deep would be better.