If you are using a 22mm surfacing bit, you can use this code for your LongMill. This code will cut 1mm down in one pass. Please set your origin to the bottom left corner of the machine.
Technically, any flat end mill or bit can work for this application. However, using a wider bit can speed up the process. In the video we are using a 22mm surfacing bit.
I used a gcode generator from INTUWiz (http://www.intuwiz.com/plane.html#.Xidt8sjYouV). Here are the settings I used for the 30×30 size:
The center of the coordinates is in a point: 1
Side a: 762
Side b: 762
Tool diameter: 22
Y overlap percentage: 45
Total depth of cutting: 1
Depth of cutting per pass: 1
Feed rate (X,Y G00): 2000
Feedrate (X,Y, G01): 2000
Feedrate( Z G00): 500
Feedrate(Z G01): 500
You can also use a different program if you want and cut a shallow pocket of the size you need.