Will:

I'm going to go out on a limb here, I'm not a slicing software programmer so this is just a guess - but this is how I'd do it. To make this easier on me were gonna make a whole bunch of assumptions:

1) We going to print a line 60mm long, 1 filament wide by .5mm tall.

2) We're going to fix the filament width in the slicer at 1mm. (Your nozzle width, which seems huge by the way.)

3) We're going to ignore acceleration and deceleration. (We've got those new reactionless drives and can go from 0-60 in nuthin' flat. We could figure in accel and decel, but it makes my head hurt thinking about it right now.)

4) We going to assume that we print nice rectangular sides on our line. They won't be, but straight sides make the math SOOO much simpler.

OK, enuff' funnin' around let's get down to the nuts and bolts. We need to print our line but before we can print it we've got to tell the printer how many mm of 1.75mm diameter filament needs to be extruded (E), and how fast to extrude it (F).

We know the volume of material that needs to be extruded, we know the length of the move, we know the layer thickness and we know the fixed filament width. For our line, this extruded volume works out to 30 cubic mm. I love nice round numbers.

We know the extruded volume, now we need to calculate the number of mm of 1/75mm diameter filament we need to get that volume. When I worked through the calculations for the volume of a cylinder 1.75mm in diameter by 1mm tall, I got 2.405 cubic mm. So our 1.75mm filament contains 2.405 cubic mm of extrudable material per mm of filament. 30 divided by 2.405 equals 12.474mm, we now know that our extrusion length (E) is 12.474mm.

We also know that we need to extrude that 12.474mm in the time it takes for the printer to get from point A to point B. With our reactionless drives on our printer we're ignoring accel and decal times, so the time it takes to get from one end to the other is just the length of the line divided by the print speed. This travel time is 60mm/(60mm/sec), or 1 second. Did I already say I LIKE nice round numbers? The feedrate (F) is the extrusion length (E) divided by travel time. Our extruder feedrate (F) works out to be 12.474mm/sec.

This may, or may not, be a very simplified version of the actual calculation. It's going to start getting hairy when you vary the filament width based on layer height like I think Slic3r does, and then add in the accel and decal times. That's just more number crunching which is what the high-speed morons that we call computers are good at.

But I'm pretty sure that it's all based on the extruded volume required by the move, and the travel time required by the move. Is that what you wanted to know?

Don