How to make sure we reach the total feedrate despite jerk When printing a G-code at a high speed/feedrate of 12,000&nbsp;mm/min versus a slower speed we don't always notice a difference since the geometry does not always allow for the full speed to be reached due to the acceleration/deceleration taking most of the length of the segments. 

Is the above a correct statement and is there a way to find the sweet spot between length and fastest speed in relation to the jerk values?
The simple answer is that there's no way to guarantee you reach a specific maximum speed, with a given acceleration limit. You have to accelerate and decelerate along the length of a line, so the distance you're accelerating is roughly half the length of the total line. Imagine you can only accelerate at 50&nbsp;mm/s2, and you've got a line that allows you to reach exactly 100&nbsp;mm/s before you start decelerating. If you have to print a shorter line, you're going to reach a lower speed. Any combination of maximum speed and maximum acceleration can be defeated just by printing a shorter line than you tested with. 

As far as a sweet spot goes, I'm not sure quite what you're asking there. There are tradeoffs to be considered for any combination of speed, jerk, and acceleration.

Prints with high jerk settings will be able to "skip" a certain portion of the acceleration and just jump straight to some minimum speed. This tends to result in echo artifacts on the print around corners.

Prints with low acceleration settings tend to exhibit some irregularities in line width over the length of the line due to the interaction between pressure and extrusion rate inside the extruder. I went into some detail on the third section of this previous answer of how and why this happens.

Prints with high top speeds, but low acceleration and jerk, can suffer from under-extrusion if you set your maximum speed too high, and it can be a little difficult to diagnose this in some cases. If you think of the extruder as having an internal "buffer" of molten plastic, that buffer tends to fill at low print speeds, and empty at higher print speeds. You may think you've got your maximum speed set appropriately, when in reality your extruder drains the buffer over maybe 50&nbsp;mm of max speed printing (hypothetical nonsensical number), and attempting to print very long straight lines may result in the extruder emptying its buffer until it slows down again. My recommendation is to do some trial and error testing to figure out the fastest extrusion rate your printer can support for an extended period of time, and then put that maximum limit into your printer firmware to prevent printing too quickly. Acceleration and jerk settings should be set as high as you can tolerate without causing echoes and other odd artifacts, with attention paid to the printer itself; it gains you nothing to shake your frame to pieces trying to chase some theoretical maximum printing speed limit.