Hard to describe, tight is tight. It's a feeling I suppose. Using a tool of any kind will apply more force than fingers so it doesn't have to be tightened to extremes. Usually you'll feel the standoff or screw 'stop' when it's tight. Use a grip on the screwdriver somewhere between "This is the only thing keeping me from falling off a cliff" and so loose you drop the driver. Shortly after you get 'feedback' it will likely stop, don't tighten it past that.
If it's 'stiff' from the beginning, stop and back off and try again. You're probably crossthreaded (not good). Just give it a couple rotations counter clockwise then attempt to screw it in (helps line up the threads). Going past the point where there's resistance would likely result in stripping them, they're fine threads and threaded into small holes (depth wise, not a lot of threads to begin with). Even using your fingertips on a screwdriver handle twisting it to tighten, at the head of the screw you're applying more force/torque than you think. It's more than just 'finger tight'.