I’m finally getting around to using Octoprint. Despite having just (re)calibrated my machine, I appear to be having a bit of over-extrusion, so I wanted to experiment on my current print by cutting the extrusion back a bit.
On the OctoPrint Control page, there’s a “Feed Rate Modifier” and a “Flow Rate Modifier”
What’s the difference? Which one would I adjust to reduce over-extrusion?
Nice! Welcome to the Pi side. 
If your using Cura, I highly recommend the Octoprint Plugin!
From my understanding, which might be wrong:
Flow rate changes the extrusion multiplier so you can change how much filament is extruded (like adjusting your esteps multiplier for extrusion). This is the one you want to adjust for over/under extrusion.
Feed rate changes the print speed, and adjusts everything else to match. So movement speed, acceleration rates, extrusion feed rate, the works.
It’s essentially the same as turning the dial on the front of the printer and telling it to work at faster speed or lower the speed. It’s great for if your seeing artifacts or need a speed change for some reason.