The approximation 10000 / diameter in mm is a good start. It works directly for HSS on steel, so a 25mm diameter steel rod would be spun at about 400rpm, and 5mm at 2500rpm. It assumes a single point tool in good order.
For other materials, halve the calculated rpm for cast-iron and stainless. Add 25 to 50% for Brass, and double it for Aluminium.
If using carbide rather than HSS, multiply the HSS result by 4 to 6. (If your machine can deliver the necessary power and speed!)
Be prepared to experiment. Much depends on what you're doing and the limitations of your set up. With the rpm in the right area, adjust depth of cut and feed rate for best results. If you get problems like chatter, poor finish, or overheating, play with rpm as well. Don't expect a hobby lathe to work at industrial production rates! I liked to run my mini-lathe so it sounded loaded without being unduly stressed, prioritising finish over metal removal rate.
Dave