It depends on the viscosity of the paint, Humbrol enamel paints require less thinner than Revell, and Pheonix paints have a special dedicated thinners which require even less thinning down. The best advice i can give is to start with 50/50, if the droplets are heavy, then more thinning is required, on the other hand if the paint is looking light and easily runs, well then it's too thin. You need to find a good balance, when you paint for a living as i do, then you will know the right viscosity just by mixxing the paint and watch it run down the side of the mixing jar.
For any over sprayed or bleeding is best taken care of before the paint has had time to fully cure, i use a lightly soaked cotten bud in thinners, then the wipe the affending area away!