Extrusion multiplier is usually recommended to be changed in measures of .05 and really not more than .1. So the first thing you have to do is get the filament size as close as possible. The slicer is going to use that in its calculation of how much filament to extrude. This affects over all extrusions.

I wouldn't worry about steps especially if using S3D or other main stream slicers since the steps for your steppers is known and the machine is known (i.e. not a home built). You can adjust with extrusion multiplier unless it is really off a lot.

I would google on using a calibration cube or thingiverse. Mainly get a 20X20X20 or 20X20X10 calibration cube and run it with default slicer settings for your machine and material (PLA) and with a couple perimeters and 100% infill. Measure the dimension and also look at the top to see if it looks concave or convex.

If the top is bowing up a bit then reduce the extrusion multiplier just a bit and run again or do the opposite if it is sunken.