The lifter has to be on the base circle of the cam, as opposed to on one of the lobes, when the valves are adjusted.
The usual suggestion is to use your fingers to spin the pushrod, and tighten the nut until you can feel "drag", and then tighten the nut the specified number of additional turns.
I prefer to "rattle" the pushrod side to side, or up and down. It's easier to tell when the slack is removed, that way. Sometimes they can spin quite easily, even when there is no slack.
The "specified number of additional turns" is either 1/2 turn, or 1.5 turns, depending upon who you want to believe. I have read both. I tend to go "looser", as long as they don't clatter when I'm finished. I think I used 1 turn, but it's been a while since I built my 3.4, so I don't remember.