There’s no formula really. Sancho is a player you have to go for. If we saw him at 16 and could get him for £25m, we should have done that too. It’s not all linear. He falls into the category of player that you try to secure ahead of your rivals at all costs.
While I want Sancho and really hope he does come (he'd improve us massively - imagine having a player on the right!!), this isn't the way we should go after the players. Mata and Sanchez were 'too good to miss out on' but we didn't need either - we literally had no position for Mata, we shunted him out on the right wing (where he's been wasted into decline ever since) and Martial was in the form of his life when Sanchez came in to his position and started playing in Pogba's, while we still had glaring deficiencies.
Whoever we're in for this summer, I hope it's for the benefit of our squad and not in fear of another team getting the player.