Dayclona is on to the fact that the type and even the brand of lifter can make a big diff. Some of the newer stuff requires less preload --that being said old days 3/4 to a full turn these days most work great with 1/2 - 3/4 turn for street.
The deal about less gives more power etc is often said yet seldom experienced. If you are racing a hyd cam you have a problem to start with IMO but...in a race engine you can set them to almost zero preload and get some more ( not much) RPM out of it-- on the street that would not be the smart move.
Looking at your set up I would say 3/4 turn and never take valve covers off ever again
The Best way is with intake off as you show--that way you can eyeball it as you do them all and get it very close. Use air to blow the oil from around the wire clip so you can really see what you are doing on each one. The ole feel it until it doesn't turn etc can be very deceptive and you learn that when you do it with intake off--some, for one reason or another do not give the right feel--you see this more with un run, un pumped lifters.
3/4 and let them rip
Hyd rollers ( most by Morel) are way less-- like a 1/4 turn and can be very sensitive to change