Layerthickness is usually about looks .3 is about as coarse as anyone goes with a .4mm nozzle .1 seems to be as fine as anyone goes... most stick with .2 height.
Infill is usually about strength. Hollow/low infill and thin walls will flex when you put the squeeze to them, the more solid you go the less it will give. PLA is super weak if you go .4mm wall thickness, think stiff plastic bag.

also... if you want it to float go closer to hollow.
There are also some slicer settings that will offset interrior surfaces by an amount. KisSlicer is one, I've heard there's nother one based in python script or something... I'm coming up short on the name atm (sorry).