You would need to set up a number of weights and way to measure the newton force being pushed back against the weight to accurately measure the hardness. You already know its 65kg which would be a medium feel memory foam along with the fact it’s 3.5cm thick. We would only recommend foam layers in 5cm and above as less than this leads to compression and flattening of the foams. It may be easier to contact the foam manufacturer to ask them what the hardness of the foam you have purchased is. It should be listed on the manufacturer’s specification sheet for memory foam.
See below as a definition of hardness.
Hardness is measured in a set range of tolerance and is not a one size fits all figure. It is worked out by compressing a piece of foam to 40% of its original size. This measurement is then displayed in newtons (N) so a hardness rating of 70-100N means it takes between 70 and 100 newtons of pressure to compress it to 40%. The higher the figure the more force is needed and, therefore, the firmer the foam.