All in the video is ok.
But sorry it is not what I'm asking for.
I need to put Frame Length column to the BOM.
It means - Not Frame length in this form: Steel 70 x 40 x 5 X 1210,00
But the Frame length in this form: 1210,00 mm
And I want to take it from the custom variable Frame Length - but the BOM doesn't know it