Moto Guzzi oil pressure relief valve building & calibrating at home without specialist equipment

'Necessity is the mother of invention' they say, and being a lifelong tinkerer, d.i.y.er and home mechanic, I've always had to be inventive and improvise. After rummaging in my 30+ years worth of spares I found most of the parts of a brand new dismantled pressure relief valve, obviously not calibrated. I don't have a compressor or air line let alone a testing manifold to screw the valve into to measure the psi at which it opens, but I do have my mum's old kitchen scales that I use mostly for weighing packages and for comparing the mass of bike bits with a view to weight reduction ("simplify and then add lightness" as Colin Chapman said, or words to that effect). having looked it up I found that the valve is supposed to open at 4 bar or in old money, around 60psi. But without 'proper' equipment how was I supposed to measure and test if this was opening at 4 bar or earlier or later? Well I applied what I could remember of some old school mathematics. The piston in the valve is nearly 11mm diameter which using old school geometry and Pi R squared gives and area of 95mm I.e. near enough 1 square cm. 4 bar is near enough 4 kg/cm squared... So by my 'calculations' I am looking for the valve to open with around 4 kg force...