When it comes to tightening taper-thread connections, you must exercise a degree of caution.
Not enough torque and the joint will leak.
But excessive torque can result in broken housings on pumps and valves – due to the force developed by the wedge action of the tapered adaptor.
This Parker document, provides a guide for torquing tapered-thread adaptors – based on the ‘turns from finger tight’ (TFFT) method:
tapered threads guide
And when it comes to taper-threads, this is about as scientific as it gets.