How can I choose between a-nis and an-drasta? Now and just now or right now seem to mean the same thing, except that English can use "right" for emphasis ("Come here right now, young man!") but, oddly, it uses "just" for the immediate past ("He was here just now, but he seems to has disappeared").
"You're right on time!" is positive, and expresses approval. "You're just in time" expresses either, "You've cut it a bit fine" or a neutral "You made it".