You see the "ratings relative to MLB" tab? Change that to whatever your lowest rookie league is and see how your scout thinks they would fare in that league. Some will claim this is "gamey" but I don't see why? You are currently looking at your scouting departments projection on how it thinks your player will end up when fully developed. No reason that scouting department can't give an opinion on how a player would do at other levels. Hell, that's part of their job. Once you get that opinion then it will be up to you to decide if that projection means they are good enough to move up or not.
Just be sure. This is the most unsure time as there are no stats go go by. Once moved up they can't be moved back
I don't think the game will do it for you if you have yourself set to do MiLB promotions and demotions unless, they reach the max IC age of 20.