I don't think there's an "optimum" value for this setting..........it's what you're prepared to put up with!
The detour factor is used to indicate how much of a detour you are willing to accept in extra movement time....and hence additional print time.....for a travel move to follow a path that takes it inside the walls of a printed part rather than taking the "as a crow flies" route of a straight line.
So, if a move in a straight line is 2mm, and you have a detour factor of 2....then you're saying that you're happy with the nozzle travelling twice as far....4mm....by staying inside the part, rather than going through fresh air. A value of 900 is virtually saying you don't care how long it takes to detour.....just don't travel over fresh air if you can avoid it! Basically, it really ought to just a tick box and not have the detour value. I use it at a similar high number...which means I really don't care how long it takes...no stringing please due to movement in fresh air if it can be avoided!