It is important that everything be tight.
But 1/2 turn beyond hand tight can be ambiguous, might need more than that or less.
Experience and feel and instinct are your guides to tightening anything, you will learn from your failure which we all have along the way.
You can almost always use caution and tighten less if you are afraid of breakage of a part.
Go back and tighten more if if needs it.
Going extra tight might be needed, or might cause something to break or get stripped.
If you ever take your tank apart to fix the leak, take the whole flush valve off again, and make sure all sealing surfaces are clean and smooth, and no nicks or burrs in the way.
There are several things you could apply to goop up the surfaces for a better seal, but you should not need them.