How to calculate load that has to be applied on the two joint surfaces
to achieve the same preload (same clamping force) as can be achieved
if joint would be tightened with the screw (torque is provided 15, 20,
30 and 35 Ncm)?

As you imply, calculations based upon the thread angle are subject to
great error due to friction between the thread surfaces and the head
seating. But for such small torque, it may be easy for you to insert a
spring between the mating surfaces and measure the compression.
Mimicking this with a load should furnish your required data.

Deformation washers are available but usually for much higher
pressures. There are also plastic deformation strips available for
this express purpose. The width of the deformed strip is matched
against a tabulation guide. Couldn?t find them with a quick search but
a ?researcher? might be more (financially) motivated.

But as you may know, ?Careless torque costs lives? -- sorry :-)

Best