Add Variable to Extrusion Amount During PRIME_BLOB macro

[Exitalterego]

Is your feature request related to a problem? Please describe

I recently changed the nozzle on my V-Core 3 from a 0.4mm nozzle to a 1.0mm nozzle due to starting a couple of large printing projects.

I started my first calibration print as normal only to discover that the prime blob with the new nozzle was far larger than expected (or really required IMO). The prime blob got to the point that the plastic was creeping up into the silicon sock on my heat block.

After doing some digging into the PRIME_BLOB macro I discovered the cause of my problem is two-fold.

Firstly, the extrusion amount is being calculated based on nozzle size:
G1 F300 E{14 / ((0.4 / nozzle_diameter) ** 2)}
But given the ** 2 in this equation, we see quadratic growth in the amount of extruded plastic. Secondly the extrusion command is issued twice.

These two issues combined result in a lot of wasted filament and potentially time spent cleaning the heat block up.

At present, I have two solutions to prevent excessive molten plastic buildup. I either disable the PRIME_BLOB by using the variable_nozzle_priming in printer.cfg or I copy all ~300 lines of the PRIME_BLOB macro into printer.cfg to change 1 line of code. Neither of these solutions is really ideal.

Describe the solution you'd like

As an improvement suggestion, if some part of that equation could be exposed as a variable to allow finer control of the extrusion amount that would probably help. For example, replacing the static squared value with a variable_blob_extrusion_factor (set to a default of 2) would allow users to fine tune the amount of filament used for the blob without sacrificing the original intention of the PRIME_BLOB code.

Describe alternatives you've considered

I initially considered requesting the whole extrusion value be simplified to a variable. However I discarded this as an option as it would overcomplicate things for most users for whom the current macro works well.

I also considered suggesting the removal of the second extrusion command, but based on the comments in the macro itself, this is here for a reason and as such should probably remain present.

Additional information

No response