weird calibration issues with modified Itty Bitty single extruder

I swapped out to a Itty Bitty but did not like how insanely hot it got. I thought I might have another go at it but with a different setup. I modified the design a bit and am using a 104 tooth big gear and an 18 tooth gear on the stepper. My thought was to give it more torque. It isn't getting hot but I cannot get it calibrated!

I am at 905 for steps/mm and when I call for 50mm it extrudes 45mm or so. I then raise it up to 910 and it over extrudes so I go to 908, it extrudes 45.5mm of ABS. I raise it up again and it under extrudes. I keep this up till eventually I am back at 910 and it is still under extruding!

I am chasing this number all over the place and I cannot get it to work out consistently! Am I up against a limit of the firmware?

Okay, so I tried the same step value and got 45mm extrusion once, and then 52mm the next time.

What is going on?

Did some experimenting and unless I first feed a small amount through (~5mm) first and then my 50mm I would get weird results. It was as if Marlin didn't know what the new value was yet. I was just using the tune menu on the LCD to change the steps. Once I got a good one I then save it in the firmware. I have the EEprom disabled as I got weird behaviors when tuning.

I know that is probably confusing but what I did to get it to work was...

Change the esteps via LCD.
Extrude 5mm via pronterface
Mark the filament at 70mm
Extrude 50mm via pronterface
Measure (should be 20mm)
Adjust Esteps via LCD if needed
extrude 5mm
Mark filament at 70
....you get the point

I had to first extrude a small amount first to load the new step value. Does that make sense?

FWIW, I found I'd get more consistent results always discarding the first extrusion. I assume it might have something to do with normalizing the temp of the aluminum block, or flushing overly soft filament out of the extruder.

Another thing we've seen is people trying to extrude faster than the extruder can keep up with, leading to inconsistent results. IIRC, 3mm/sec for 1.75mm filament and 1.5mm/sec for 3mm filament are about the upper end.

So I finally found a number for esteps that works. I am getting consistent 50mm extrusions.

However, my prints are over extruding...a lot! I have to adjust the flow by almost 75% to get it right. I am not using a multiplier in the slicer and I have the diameter set at 2.85, which is the diameter of the filament.

If I then multiply my esteps by the multiplier and run the print again it under extrudes. I can't remember having this much trouble with the stock extruder. Could something be off with he physical setup that I am missing? I though I could simply use any belt/pully combination as long as the estep value is correct.