I just finished the assembly of my first printer (Mendel90 completely self-sourced) and after many days of reading howto's, watching youtube vids and so on I arrived on the last few things before start printing.
A good calibration is a must and I used Triffid's guide. All went well until I tried to compensate the e_steps:

new_e_steps = old_e_steps * (100 / distance_actually_moved)

Fully agree e.g. 105mm moved means the new_e_steps will be lesser than old_e_steps = 0.952

but

 … or, old_e_steps * (100 / (distance_to_mark + 80))

is not correct. You should measure the distance to the 120mm mark from end of the extruder housing. In case of 105mm moved this is 15mm. Put into the formula you get 1.052

If I am not wrong I would like to correct this way:

 … or, old_e_steps * (100 / (120 - distance_to_mark))

calculated: 100 / (120 - 15) = 0.952

Does anybody agree?

Yes, you are correct. The distance actually moved would be (120 - distance to mark), not (distance to mark + 80)