As per our PM’s, what you are doing is drawing an object and then trying to say it weighs 5kg ( which is OK but you can’t expect the math to work in that way)…The object needs to be scaled to fit its weight.
A solid disk of cast iron needs to be 159.5mm x 35mm to weight 5kg
if you draw up a cylinder with these dimensions and calculate the mass it will show 4.99kg in the solid body slot…
Like I said in the PM…If you want this object to fit a certain weight you are going to have to do ALL the calculations…I am not sure if it could be done with a ratio or not…I haven’t taken math for 40 years…
Is where I got the size of a disk to find the radius…I left 35mm thick and adjusted the diameter till I got 11.2lbs =5kg
Then in Blender…made a cylinder to those dimensions…
Select the disk and add the ridged body… don’t enter anything in the Mass
Bring up Calculate Mass-Cast Iron…and check the KG in the soft Body…4.99Kg
PROBLEM !! The object is a batch of different parts…you have to subtract all those volumes from the overall and still try to keep 5Kg ???
( I only have half the math skills you would need to do that!! )