My rule is as loose as possible without skipping during hard acceleration and braking. Overtightening will lead to the motor working harder than it has to, meaning more heat and stress. Wider belts can afford to be looser than thinner belts.
I had some problems with the motor mount it would get loose and then my belt would get loose so now i tried to tighten it a little bit more but now that i think was a bad decision … ok thanks ( @Jinra you are everywhere all the time ?:)) thank you for all the help really really appreciate it)