OMG You are a GOD!!!!!! This answer should be pinned in golden/fire letters in the forum!!!!
THANKS THANKS THANKS!!!!
I have been struggling with tinkercad, affinity designer, inkscape, code editors... maybe 24 hours in 3 days.
At first I could not import to tinkercad : (see image attached). And It was a 24x 123 mm "piece" (but the file was 300 dpi, as before I used to print it in cardboard to make a template to cut leather ).
After searching on google and reddit a lot I found a semi workaround using inkscape (and resizing in tinkercad to get it the right size but my "OCD" nedded a "cleaner" solution )and then I came here and found this:
So I started playing with the svg code in a code editor.... and changing units, the viewbox,etc (I even started reading the fracking svg specification and I´m not a coder) Some advancemets were made but the dimensions always were a bit off when importing to tinkercad and then your magic number !!!!! 25.4 dpi
I even was about to file a bug report!!!!
PD: Now my OCD wants to be able to make a custom preset with this DPI but the 25.4 gets rounded to 25 when I create it!!!!!! GRRRRRRR