Welcome to the SeeMeCNC Community

so, I have generally been doing <0.2 layers, and they look good, flat. but i recently tried to do 0.4 layers with a 0.5 nozzle and the rows of plastic were spaced very far apart. I could fit my finger nail between the rows. after some research, it seems like this can be addressed in your slicer with either:
1) increase flow rate, or
2) decrease the nozzle size

How do you address this issue? change one setting a lot, both settings a little? is there a third option?

What size is your nozzle? 0.35 won't (AFAIK) print much thicker than 0.25mm without serious problems. Similarly, a 0.5mm nozzle won't get much wider than 0.4mm without issues.

You want at least a 1.5:1 W/H ratio for layers to bond, so at 0.4mm layer height you need to set the extrusion width to at least 0.6mm, and you'd probably be better off with 0.7mm, if you are using Slic3r it always used to default wider that for a 0.5mm nozzle (but you should probably check the output), Cura and KissSlicer require you set it explicitly.

0.4mm is about as high as you can go with a 0.5mm nozzle.