when promising starts I know you have to start the player 90 percent of the time or something like that. With minutes is this also true? Does offering 20 mins equate to at least 18 mins?

Sometimes. Each player has their own threshold for what is acceptable to them. In general, 80% of the starts or minutes promised works. I've seen players transfer when averaging 19mpg on a promise of 20. Conversely, I've promised a kid a start and minutes only to leave him off of my depth chart completely and had him log 0 minutes for the season and not only did his WE never drop but he didn't transfer.