One of the main variation of this metric from other traditional ones is that only the defending players who are in the scope of the pass are considered for packing and not all the defenders on the pitch. The other difference would be the fact that for defenders, their lines on the pitch are considered with respect to the pass direction. Refer to the examples on more details about these differences.
If a defender’s line is cut by a pass, they’re considered to be packed (+1 for that defender) even if the defender is not near the line of pass (pass is on one side of the pitch and defender is far away).So if an attacking player makes a long pass beyond the entire defense, then all the defenders would be considered as packed (+1), but in truth only few of the players would be near the line of pass and could have an impact on the outcome of the pass/play.
Also for different types of passes (forward, back & side) packing is still calculated but a constant is multiplied with the packing value based on the type of pass.
For a back pass, multiplying factor is
For a side pass, multiplying factor is
For a forward pass, multiplying factor is
This is still a work in progress as there are certain edge cases where packing rate could be incorrect. Please provide a feedback/comment on Twitter/GitHub if you encounter any errors.