How tight's going to depend on the cut of the sail, & especially how baggy it is. You don't want the leech fluttering, as that'll destroy the sail (how fast depends on wind
speed). But creating a cup in the leech, because the line's too tight will slow you down. Not to mention; look fugly, & make it harder to trim the sail properly, as the cup will affect the tell tales from flying properly.
Some of how often you need to adjust it, again, goes to the cut & bagginess of the sail, & how much it changes shape when you change the main's trim settings. And or angle to the wind
Also, it's likely that it'll need tuning when you reef. Although with this, to some degree it depends upon how savvy the sailmaker
was, & what little tricks they may or may not have used when the sail was made. So that there's less need to adjust it when tucking in, or shaking out a reef.
On the fact that there are no blocks there for adjustment. Depending on the sail's size; you can add some blocks, add a small cleat for the line, or add a double sided Velcro tab like on a surfboard leash's cuff.
The part which attaches to the leech line is typically attached to a 2" wide tab, say 9"-12" long, & has Velcro "hooks" on one side, & "loops" on the other. And the sail has a corresponding Velcro panel on it, as well as a 12" flap with the other "side" of the Velcro on it.