If you get 1 inch of travel for every 3 revolutions of the motor and the maximum motor speed is 1200 RPM then divide 1200 RPM /3 Revolutions per inch and you get 400 inches per minute for a 1:1 or direct motor to screw coupling.
Now look at the the maximum speed that you can turn the ACME or ballscrew and stay below the max. speed for the screw. Adjust your calculations.
You also need to consider torque when deciding on a gear ratio or coupling ratio.