I like QCAD more and more as I use it
and become more trained.
The mistake was done because I was
thinking the Microsoft way (fuzzy logic) of doing it.
It is better to use clear logic thinking when
working with QCAD and the work will go smooth.
In this case click on "from liine" and then on
"to line" and at last position the dimension line
and make the third click. Done.