New 12" user and I am LOST!

[printbus]

So I can get in and change it on the EEPROM and save it there without messing with the arduino side of things? basically the same thing correct?

Yes - that is correct. On the LCD, setting CONTROL | MOTION | | Esteps/mm is exactly the same thing as changing the fourth constant in configuration.h DEFAULT_AXIS_STEPS_PER_UNIT. I ran my printer this way for several months, staying away from the Arduino side on purpose. You can keep doing that - you just have to remember that every time the printer logic is reset, it reverts back to what ever parameter it has been hardcoded to. The in-between approach is that you allow EEPROM updates to be possible from the LCD. Takes a firmware update to do that. But then once you have parameters set via the LCD the way you want them, you can store the parameters in to a non-volatile memory space on the MEGA2560 board. After every reset, Marlin will then use those parameters instead of the hardcoded firmware ones.

My understanding is if you are using a different roll a lot you can setup a different config file with a extrusion multiplier and this can accomplish the same thing a EEPROM save can, just different for different rolls of filament.

From the slicer perspective, yes. What we're managing here is the volume of filament flowing onto the bed. There are multiple configuration parameters that feed into that. In theory, you can adjust them all. First there's the "steps per unit". This defines the rate that the extruder stepper motor needs to rotate in order to push 1mm of filament into the hot end. The planning math in Marlin calculates what length of filament it needs in order to obtain the extrusion output it wants. There's essentially a gear ratio factor involved in this - the DEFAULT_AXIS_STEPS_PER_UNIT in configuration.h or the CONTROL | MOTION | steps/mm factors on the LCD. If the gear ratio is wrong, more or less filament will be pushed than is expected. As mentioned earlier, the MakerFarm firmware defaults seem to always be low for everyone who has reported calibration results here, with a symptom that the output extrusion will be on the light side. This is because the filament isn't actually being pushed into the hot end at the linear rate that the Marlin firmware is assuming.

Then in the slicer, there's the filament diameter. Even though the linear rate of movement might have been corrected per the above, the amount of filament volume going into the hot end will vary if the filament diameter varies. If the slicer is configured for 1.75mm diameter filament and you're actually using 1.68mm diameter filament, the slicer will actually be commanding less filament volume to be pushed into the hot end than it should, with again, light extrusion being a symptom. Again, this is in the slicer. Marlin firmware doesn't know or care what size filament you're using.

Then there's the extrusion multiplier. That basically tells the slicer that OK, regardless of any corrections made to the rate we're commanding the stepper motor to feed the filament and the diameter of the filament, apply yet another correction to how much filament is actually being extruded.

Filament diameter and extrusion multiplier can be saved on a filament roll by filament roll basis, and many people do that.

There are even more ways to modify the rate that filament is being extruded at print time, but I'm not going to go into them at this point since they'd likely add to your confusion.

As a bottom line, all of these parameters can and will affect the volume rate that filament is extruded. Each just does it in a different way. I'll stick my neck out and say there's no right or wrong way here - you pick how you want to do it and stick with it. I found that my steps per unit varies little, so I freeze that as part of firmware, rechecking the value every blue moon or so. I configure the slicer for the filament diameter every time I change the filament. I typically leave the extrusion multiplier at 1.0, likely mainly since I've never taken my calibration far enough to care about that additional detail - my prints seem good enough after setting the other two parameters. In the odd cases where I'm in the middle of a print and I wish I was extruding a bit more or less, well, that can be tweaked from the LCD.