Rega Apheta 3 on Rega Planar 8:  Any idea on screws torque values ?

When it comes to cartridge bolts and torque - less is more.

You should just nip them up gently - too much torque can distort the cartridge body and impact sound negatively - at worst damage the cartridge.

Per Rega the Aphelion 2 torque is (0.4 Nm)

You might want to ask the distributor in your country (or closest country) for info on the Apheta 3. 

Here is a list of global Rega distributors: Rega Dist

depends on the cartridge body undergoing deformation under load . Common sense is a good guide..