The lenght of the spreaders and standoffs is measured for using r-sky cross (6x6 mm, not 6x7 mm) so with 6mm at the ferrule/spreader side and 6mm at the spine. They don't go inside the cross, only the ferrule goes in. So Ion is right: from the edge of the cross.
When you use a cross where the rods go inside and touch each other in the middle like your last picture you have to add 6/7 mm to the length of each spreader in order to keep the dimensions the same.
Thanks karel_oh for the clarification
Note that I also reduce the length of the standoffs by 6 mm because the cross I use is shorter than the R-Sky cross.