Yea, you want to be a little careful when measuring/marking 100mm then attempting to extrude that much. When it’s “short”, it’s fine, but if it’s “long”, there’s no way to see (assuming you are measuring from the top of the module like i did).
If you use something larger than 100mm (like 120 or 150) for your markings, you then still attempt to extrude 100mm, you should still be well within ease of measuring.
And just to be clear, each new calculation uses the new E value. it looks like 2Bpencil’s calculations are doing that, though it took a minute to determine that “left of” was under extruding and “more of” was over extruding. Those values are “off” though as you can see in the latest post because of rounding.
So i use a program call SpeQ Mathematics, which allows you to do all kinds of stuff, like input formulas. You can see here a = E, b = expected extruded value, c = remaining distance to extruded value (so “past it” being negative). So using 2Bpencils’ value of 249.55 is what got closer to 237 for the next E by setting the negative 5 for the “distance”. Again just to help people understand what’s going on.
But yea, you shouldn’t round when you don’t have to, esp not in a formula. So the initial value you should have gotten with 15mm left is 249.66 for your next E @2Bpencil (not 249.55).
I can say after working with mine, then redoing it after swapping hot ends (and finding it over extruding slightly) that if you’ve been printing for a bit, check to see if your nozzle is actually extruding properly. it could have hardened bits in it, the tip could not be proper anymore (esp if you’ve gouged the plate every now and then), bring it up in temp a bit to make sure everything melts properly, make sure the filament isn’t getting “snagged” on anything as it’s being pulled from the spool, etc.
For the stuff I print though, it’s not for any type of business purposes, so as long as it’s “close enough” that’s good enough for me. Hell i have been printing things for a long time without doing this and those models were fine! It can matter which filament you use too. the “better” filaments tend to melt and flow easier, making this less noticeable.