"That will be just right!"
Translation:Det blir lagom!
You have to be careful with ska! It does not mean "will" as one might guess. It entails more than that and shows much more intention and decisiveness.
We usually use the present tense to talk about the future. "Det ska vara lagom" sort of feels like your forcing someone to accept that it should be "just right".
That's sort of borderline. It's pretty clear that ska vara would mean it's supposed to be or it should be in English. Since with bli, there's at least a change of state, it's conceivable that it might work. It could be a promise: I promise that it will be the right amount. This meaning doesn't work with vara because I can't control what is "enough" – I can't change the level required for something to be enough, but maybe I can control the amount of something, so I can make it "be" enough in the sense make it "become" enough (i.e. increase or decrease the amount until it's enough). In many cases, the difference between being and becoming is more important in Swedish than in English.
tl;dr; yes and no, there's a narrow sense where that might work but it's not what this sentence is most likely to mean.
That's correct, but it's in the sense of "no more than that", whereas lagom is more like "just the perfect amount". In a sense, "enough" describes a constraint, and lagom an interval.
Also, ska is a modal so it'd be det ska räcka, and bli doesn't take verbs so that construction wouldn't work for that reason either.